好做手腳 Good Light For Sleight Of Hand
行者估著來擡他，他就脫身道：「此燈光前好做手腳。」拔下一根毫毛，吹口仙氣，叫：「變！」即變做一個行者，綑了麻繩。行者估着来抬他，他就脱身道：「此灯光前好做手脚。」拔下一根毫毛，吹口仙气，叫：「变！」即变做一个行者，捆了麻绳。
逐句解 Line by line
Pilgrim reckoned they were coming to lift him next, and slipped free, saying, "This lamplight is good for a bit of sleight of hand."
做手腳, to work the hands and feet, means to pull a trick or tamper with something, and it is still current in that sense. 著 here is zhe, marking the reckoning as something he goes on doing while he waits.
He pulled out a hair, blew a magic breath on it, and cried, "Change!"
The oldest trick in his repertoire and the book never tires of it. Twelve characters for the whole operation: pull, blow, shout.
It turned at once into another Pilgrim, bound about with hempen rope.
The detail that makes it work is 綑了麻繩: the double comes ready-tied. A copy that arrived free would be no use at all.
The lamplight that let Tripitaka see his bound disciples is now working for the other side. Wukong needs to be watched in order to be swapped, and he says so out loud.
The hair trick has been in the book since the early chapters, and it is worth noticing how casually it is deployed. No incantation, no preparation. He has been lying in a heap on the floor of a demon's hall for half the night and the escape costs him one hair and three words.
The copy arrives already bound, which is the piece of craft. Wukong is not producing a duplicate of himself; he is producing a duplicate of himself as the demons last saw him, and that is a different and much harder thing.