一根毫毛 A Hair Pulled From His Nape
好大聖，他也不使長老知道，悄悄的腦後拔了一根毫毛，吹口仙氣，叫：「變！」即變做本身模樣，陪著沙僧，隨著長老。好大圣，他也不使长老知道，悄悄的脑后拔了一根毫毛，吹口仙气，叫：「变！」即变做本身模样，陪着沙僧，随着长老。
逐句解 Line by line
Fine Great Sage. Without letting the elder know, he quietly pulled a hair from the back of his head, blew a breath of immortal air on it and cried, "Change!"
好大聖 for the third time in this run of the story, and the third different kind of act it introduces: a flight, a decision, and now a deception. 不使長老知道 — without letting the elder know — is stated as the first thing about the trick, before the method.
It turned at once into a likeness of himself, to keep Sha Monk company and attend the elder.
模 is mú in 模樣, a likeness. Both 著 are zhe, actions continuing: the double keeps company and goes on keeping company. What stays behind is a hair with Pilgrim's face on it, and the two it is minding do not know.
The hair trick is old and this is a new use for it. Usually it makes a decoy for an enemy; here it makes one for his own master, so that nobody at camp notices he has gone.
He is going to rescue his brother, which is the good thing to do, and he is doing it in a way that leaves no witness. Both of those are true at once and the novel does not sort them out.