He made the sign, spoke the words of power, gave his body a shake, and turned into a lamp-diving moth:

捻著訣 is working the hand-sign, fingers folded into a prescribed shape — the 著 is zhe again, the action held while the next one happens. 真言 is the true word, the standard rendering of mantra. The moth is named for what it does: dives at lamps.

Slight of body, its wings thin and nimble; it puts out lamps and beats at candles, throwing itself towards the light.

The verse changes shape here, and it is worth counting. Six characters, six, seven, six — and then the same four lengths again, fifty characters in all. That is a song lyric written to a fixed tune, not a poem in regular lines, and it gives the passage a lighter, faster tread than the city verse a moment ago. 投明 is to throw oneself at the light, and it is doing the work of the whole passage: this is a creature defined by heading straight for the thing that will kill it.

Its true face came into being by transformation.

本來面目 is a Buddhist term of art — original face, what you were before anything happened to you — and it is still the phrase Chinese reaches for when someone's real character shows. Applied to a moth it is quietly funny, and applied to Wukong in a moth's body it is funnier.

Out of rotting grass its quickening answers.

Old Chinese natural history held that fireflies are generated out of decaying grass, and the belief is old enough to be recorded in the ritual calendar. 應 is yìng, to answer or respond — not the reading that makes it a modal. 靈應 is the responsive spark, the animating power arriving where nothing had been.