The narrator steps forward here to explain the trick, and it is worth asking why. The demons believe a cord was cut from outside, which is impossible, and the novel does not want the reader believing it too. So it stops the action and says plainly: there was never a cord to cut, only a hair, and taking the hair back ends the matter.

The phrase for it means something like a method that hides its own working, which is a fair description of most of what Wukong does. His transformations and duplications are frequently less about power than about making people believe a situation is other than it is. Here the whole hold he had on the demon was a piece of hair and a confident manner.

Notice too that Wukong examines the promise before releasing it. The novel gives him a verb of judgment and tells us he found the undertaking honest. He is wrong, as the rest of the chapter will show, but the mistake is not carelessness. He checked, and the check was passed.