避火訣 The Fire Averting Charm
八戒笑道：「你常說雷打不傷，火燒不損，如今何又怕火？」行者道：「你這獃子，全不知事。那時節用心防備，故此不傷；今日只為搧息火光，不曾捻避火訣，又未使護身法，所以把兩股毫毛燒了。」八戒笑道：「你常说雷打不伤，火烧不损，如今何又怕火？」行者道：「你这呆子，全不知事。那时节用心防备，故此不伤；今日只为扇息火光，不曾捻避火诀，又未使护身法，所以把两股毫毛烧了。」
逐句解 Line by line
Eight Rules said with a laugh, 'You are always telling us that thunder cannot hurt you and fire cannot harm you. How is it you are afraid of fire now?'
雷打不傷，火燒不損 is a matched pair. Eight Rules has caught a real inconsistency, and for once the novel lets him land it.
Pilgrim said, 'You idiot, you understand nothing at all.'
獃子 with the light ending, as always from Pilgrim. 全不知事, "wholly ignorant of how things are" — the abuse comes first and, unusually, an actual explanation follows.
'On those occasions I was guarding myself with care, and so took no hurt; today, because I was wholly taken up with fanning the fire out, I never made the fire-averting sign nor used the body-protecting spell, and that is why the hair on both my legs got burnt.'
曾 is céng, marking something already done. 捻…訣 is to make a hand-sign, to work a charm with the fingers; 避火訣 is the charm against fire. 為 is wèi. The source runs the two halves together and they are rejoined here: 那時節 against 今日, each with its own reason and result, strictly parallel and opposed rather than climbing.
Eight Rules catches a real inconsistency and the novel lets him have it. You are always saying thunder cannot hurt you and fire cannot harm you — 雷打不傷，火燒不損 — so why are you afraid of fire now? It is a fair question, put in a matched pair of four-character clauses, and it is the only time in the chapter anyone successfully challenges Pilgrim on a point of fact.
The answer is technical and it is convincing. On those occasions I was guarding myself; today I was wholly taken up with fanning, made no fire-averting sign and used no body-protecting spell. His invulnerability is not a property he has but a practice he performs, and it takes attention he was spending elsewhere.
That is a more interesting proposition than it first looks, and it holds throughout the novel. Pilgrim is nearly always hurt when he is distracted, over-confident, or busy with something else — and here he was concentrating on the very thing that was supposed to solve the problem, with a fan that was making it worse.