I’ve written several articles on Sun Wukong’s iron staff, including its origin from religious and martial staves used by historical Buddhist monks, the meaning of its inscription (“‘As-You-Wish’ Gold-Banded Cudgel. Weight: 17,550 lb” (7,960.54 kg)), the real world metal that it is made from, its ties to Yu the Great and flood control, its ties to the Buddhist Saint Mulian, its possible ties to a Hindu monument, and modern day misconceptions about its ability to weigh down the entire Milky Way galaxy. Now, I’d like to briefly survey the magic powers associated with this weapon. This will by no means be exhaustive.
A. Size Manipulation
Sun travels to the Eastern Sea Dragon King’s underwater kingdom in ch. 3 to acquire a celestial weapon. But when the immortal fails to find a suitably heavy armament, the Dragon Queen suggests that they give him a black iron pillar from their treasury. It is described as 20 feet (6.096 m) in height and the width of a barrel. Only when Monkey lifts the pillar and suggests a smaller size would be more manageable does it comply with his wishes:
Wukong girded up his clothes and went forward to touch it: it was an iron rod more than twenty feet long and as thick as a barrel. Using all his might, he lifted it with both hands, saying, “It’s a little too long and too thick. It would be more serviceable if it were somewhat shorter and thinner.” Hardly had he finished speaking when the treasure shrunk a few feet in length and became a layer thinner. “Smaller still would be even better,” said Wukong, giving it another bounce in his hands. Again the treasure became smaller. Highly pleased, Wukong took it out of the ocean treasury to examine it. He found a golden hoop at each end, with solid black iron in between. Immediately adjacent to one of the hoops was the inscription, “The ‘As-You-Wish’ Gold-Banded Cudgel. Weight: 17,550 pounds [Ruyi jingu bang zhong yiwan sanqian wubai jin, 如意金箍棒重一萬三千五百斤]” (Wu & Yu, 2012, p. 135). 
Later in the chapter, Sun shows off the new weapon to his children by shrinking it to the size of a needle and then expanding it to a literal pillar of heaven.
He held the treasure [the staff] in his hands and called out, “Smaller, smaller, smaller!” and at once it shrank to the size of a tiny embroidery needle, small enough to be hidden inside the ear. Awestruck, the monkeys cried, “Great King! Take it out and play with it some more.” The Monkey King took it out from his ear and placed it on his palm. “Bigger, bigger, bigger!” he shouted, and again it grew to the thickness of a barrel and more than twenty feet long. He became so delighted playing with it that he jumped onto the bridge and walked out of the cave. Grasping the treasure in his hands, he began to perform the magic of cosmic imitation. Bending over, he cried, “Grow!” and at once grew to be one hundred thousand feet tall,  with a head like the Tai Mountain and a chest like a rugged peak, eyes like lightning and a mouth like a blood bowl, and teeth like swords and halberds. The rod in his hands was of such a size that its top reached the thirty-third Heaven and its bottom the eighteenth layer of Hell (fig. 1) (Wu & Yu, 2012, vol. 1, p. 138).
B. Controlling the oceans
Prior to giving Monkey the staff, the Dragon King tells his wife, “That…was the measure with which [Yu the Great] fixed the depths of rivers and oceans when he conquered the Flood” (Wu & Yu, 2012, vol. 1, p. 135). Later, in ch. 88 our hero recites a poem in which he gives more detail about the weapon’s origins and history. The first few lines discuss its power over water:
An iron rod forged at Creation’s dawn
By Great Yu himself, the god-man of old.
The depths of all oceans, rivers, and lakes
Were fathomed and fixed by this very rod.
Having bored through mountains and conquered floods,
It stayed in East Ocean and ruled the seas,
[…] (Wu & Yu, 2012, vol. 4, p. 201).
Despite the staff’s influence on bodies of water both great and small, it paradoxically doesn’t grant Sun an advantage when traveling through the aquatic realm or fighting water-based demons.  I’ll just chalk this up to inconsistencies born from oral storytelling.
C. Astral entanglement
Ch. 3 shows that Monkey’s soul is able to use the staff in the underworld even when the physical weapon is back with his body in the world of the living.
In his sleep the Handsome Monkey King saw two men approach with a summons with the three characters “Sun Wukong” written on it. They walked up to him and, without a word, tied him up with a rope and dragged him off. The soul of the Handsome Monkey King was reeling from side to side. They reached the edge of a city. The Monkey King was gradually coming to himself, when he lifted up his head and suddenly saw above the city an iron sign bearing in large letters the three words “Region of Darkness.” … Yanking and pulling, they were determined to haul him inside. Growing angry, the Monkey King whipped out his treasure. One wave of it turned it into the thickness of a rice bowl; he raised his hand once, and the two summoners were reduced to hash.
[After reprimanding the 10 judges for bringing his soul to hell, Sun says,] “All I want is to erase my name [from the ledgers of life and death]. Bring me a brush.” The judge hurriedly fetched the brush and soaked it in heavy ink. Wukong took the ledger on monkeys and crossed out all the names he could find in it [fig. 2]. Throwing down the ledger, he said, “That ends the account! That ends the account! Now I’m truly not your subject.” Brandishing his rod, he fought his way out of the Region of Darkness.
While our Monkey King was fighting his way out of the city, he was suddenly caught in a clump of grass and stumbled. Waking up with a start, he realized that it was all a dream (Wu & Yu, 2012, vol. 1, p. 139).
Sun’s ability to use the weapon as a disembodied spirit implies that it has some power of astral projection and entanglement (i.e. it goes where his soul goes). However, to my knowledge, this only happens once in the story, and the novel clearly demonstrates that he can’t use the weapon if it is physically taken away from him.  This is likely another inconsistency from oral storytelling.
The weapon is shown capable of creating manifold copies of itself. For example, in ch. 4, Monkey multiplies his staff to accommodate his monstrous, multi-armed form while fighting Prince Nezha: “Dear Great Sage! He shouted, ‘Change!’ and he too transformed himself into a creature with three heads and six arms. One wave of the golden-hooped rod and it became three staffs, which were held with six hands” (Wu & Yu, 2012, vol. 1, p. 155). Later in ch. 50, he rains staves down on a demonic army.
Using the tip of his lance to point at the ground, the demon king shouted for the little imps to attack together. All those brazen fiends, wielding swords, scimitars, staffs, and spears, rushed forward at once and surrounded the Great Sage Sun completely. Entirely undaunted, Pilgrim only cried, “Welcome! Welcome! That’s exactly what I want!” He used his golden-hooped rod to cover his front and back, to parry blows east and west, but that gang of fiends refused to be beaten back. Growing more agitated, Pilgrim tossed his rod up into the air, shouting, “Change!” It changed immediately into iron rods by the hundreds and thousands; like flying snakes and soaring serpents, they descended onto the fiends from the air” (Wu & Yu, 2012, vol. 2, p. 372).
Sun demonstrates the staff’s magic lock-picking ability in ch. 25.
The doors are all locked. Where are we going to go?” “Watch my power!” said Pilgrim. He seized his golden-hooped rod and exercised the lock-opening magic; he pointed the rod at the door and all the locks fell down with a loud pop as the several doors immediately sprung open. “What talent!” said Eight Rules, laughing. “Even if a little smith were to use a lock pick, he wouldn’t be able to do this so nimbly.” Pilgrim said, “This door is nothing! Even the South Heaven Gate would immediately fly open if I pointed this at it!” (Wu & Yu, 2012, vol. 1, pp. 468-469).
Admittedly, this passage could be read two ways: 1) The staff opens the lock; 2) Monkey uses the staff as a conduit for his own lock-picking magic. But I’m choosing the first interpretation as this ability was likely influenced by Saint Mulian unlocking the gates of hell with his staff. 
In ch. 46, during a competition of Buddhist and Daoist prognostication, Sun magically disguises himself as a Daoist lad’s ritual master and convinces the boy to let him shave his head: “He changed his golden-hooped rod into a sharp razor, and hugging the lad, he said, ‘Darling, try to endure the pain for a moment. Don’t make any noise! I’ll shave your head.’ In a little while, the lad’s hair was completely shorn” (Wu & Yu, 2012, vol. 2, p. 305). In ch. 65, Sun turns the staff into a drill in order to escape from a pair of magic cymbals, using the tool to bore a hole in the horn of a dragon that was just able to pierce the seam: “Marvelous Great Sage! He changed the golden-hooped rod into a steel drill and drilled a hole on the tip of the horn. Transforming his body into the size of a mustard seed, he stuck himself inside the hole and yelled, ‘Pull the horn out! Pull the horn out!'” (Wu & Yu, 2012, vol. 3, p. 218).
When the Dragon Queen originally suggests giving the pillar to Monkey, she tells her husband: “These past few days the iron has been glowing with a strange and lovely light [fig. 3]. Could this be a sign that it should be taken out to meet this sage?” (Wu & Yu, 2012, vol. 1, p. 135). This might imply the weapon was aware of its new master’s imminent arrival. Later in ch. 75, Sun recites a poem speaking of the staff’s desire for flight.
Its name was one Rod of Numinous Yang,
Stored deep in the sea, hardly seen by men.
Well-formed and transformed it wanted to fly,
Emitting bright strands of five-colored mist.
Enlightened Monkey took it back to the mount
To experience its pow’r for boundless change.
[…] (Wu & Yu, 2012, vol. 3, p. 375).
The phase “wanting to fly” (yao feiteng, 要飛騰) could be read as a metaphor for yearning to be released from the dragon treasury and/or a call for adventure. Add to this the staff’s ability to follow Sun’s wishes to grow, shrink, multiply, change form, and pick locks. Therefore, the novel depicts the staff having a certain amount of awareness. 
Fig. 3 – Monkey pointing to the luminous iron pillar (larger version). From the Qing-Era Painted, Complete Edition Journey to the West (Qing caihui quanben Xiyouji, 清彩繪全本西遊記).
Journey to the West (1592) describes the Monkey King’s iron staff having the magic power to shrink and grow, control the ocean, astral project and entangle with Monkey’s spirit, multiply endlessly, pick locks, and transform into various objects. It also has sentience to a certain degree.
1) I have changed Yu’s (Wu & Yu, 2012) dry rendering “Compliant Golden-Hooped Rod” to the more pleasant one based on W.J.F. Jenner. Also, Yu’s original translation says “13,500 pounds”. However, the Chinese version uses jin (斤), known in English as “catty“. The catty and pound are two different measures of weight, the former being heavier than the latter. Therefore, the English text has been altered to show this. The catty during the Ming Dynasty when the novel was compiled equaled 590 grams (Elvin, 2004, p. 491 n. 133), so 13,500 catties would equal 17,550 lb.
2) Here, Yu’s (Wu & Yu, 2012) English translation says Monkey grows to be “ten thousand feet tall”. However, the original Chinese source reads “萬丈” (wanzhang), wan meaning 10,000 and zhang being a measure designating 10 Chinese feet (10,000 x 10 = 100,000). Therefore, I have changed the source to read “One hundred thousand feet”, much like Yu (Wu & Yu, 2012) translates it in chapters six (vol. 1, p. 181) and 61 (vol. 3, p. 157).
“Worthy Brother,” said Pilgrim with a laugh, “in this case I’ve really nothing to brag about, for I’m just not comfortable doing business in water. If all I do is walk around down there, I still have to make the magic sign and recite the water-repelling spell before I can move anywhere. Or else I have to change into a water creature like a fish, shrimp, crab, or turtle before going in. If it were a matter of matching wits in the high mountains or up in the clouds, I know enough to deal with the strangest and most difficult situation. But doing business in water somewhat cramps my style!” (Wu & Yu, 2012, vol. 1, pp. 423-424).
4) The rhinoceros demon sucks it away with Laozi’s magic steel bracelet in ch. 50 and 51. A lion spirit uses a magic wind to steal the weapons of all three pilgrims in ch. 88. In both cases, Monkey resorts to trickery to retrieve the physical staff from their respective mountain strongholds.
5) One section of Mulian’s tale reads: “With one shake of his staff, the bars and locks fell from the black walls, / On the second shake, the double leaves of the main gate [of hell] flew open” (Mair, 1994, p. 1113).
6) The idea of sentient weapons is certainly not unique to Journey to the West considering that the ancient Chinese ascribed souls to noted swords. For example, Yuan poet Jia Penglai (賈蓬萊, c. mid-14th-c.) described famed Spring and Autumn period blacksmith Ou Yezi‘s (歐冶子) treasure swords Longyuan (龍淵, a.k.a. Longquan, 龍泉) and Tai’e (泰阿/太阿) as mated lovers who pine for each other when separated and even leap from the scabbard to seek out their beloved (Lee & Wiles, 2015, pp. 161-163).
Elvin, M. (2004). The Retreat of the Elephants: An Environmental History of China. New Haven (Conn.): Yale university press.
Lee, L. X. H., & Wiles, S. (2015). Biographical Dictionary of Chinese Women: Tang Through Ming: 618-1644. Abingdon: Routledge.
Mair, V. H. (1994). Transformation Text on Mahamaudgalyayana Rescuing his Mother From the Underworld With Pictures, One Scroll, With Preface In V. Mair (Ed.), The Columbia Anthology of Traditional Chinese Literature (pp. 1094-1127). New York: Columbia University Press.
Wu, C., & Yu, A. C. (2012). The Journey to the West: Vol. 1-4. Chicago, Illinois: University of Chicago Press.