伏之於地 Flat Upon The Ground
慌得那八戒戰戰兢兢，伏之於地，把嘴拱開土，埋在地下，卻如釘了釘一般。沙僧蒙著頭臉，眼也難睜。慌得那八戒战战兢兢，伏之于地，把嘴拱开土，埋在地下，却如钉了钉一般。沙僧蒙着头脸，眼也难睁。
逐句解 Line by line
It threw Eight Rules into such a fluster that he shook from head to foot, dropped flat upon the ground, rooted a hole in the soil with his snout and buried himself in it, as fast as a nail driven home.
The simile hangs on two readings of one character. 釘了釘 is dìng le dīng: the first is the verb, to drive a nail, and the second is the noun, the nail itself. The 1716 dictionary keeps the two apart under separate spellings, and its gloss on the verb is literally "to fasten a thing with a nail" — the very construction used here, with the order reversed. Automatic transcription gives the noun reading twice and loses the point.
Sha Monk covered his head and face and could scarcely open his eyes.
著 is the marker zhe. 難 is the adverb nán, "hard to do."
Prose that follows a recitation cannot belong to it, so this stands apart — and the separation does the poem a favour. The quatrain ended on darkness over a whole district; the next thing described is a pig's snout in the dirt.
Eight Rules buries himself using the equipment he has, which is what he always does. The simile is domestic and exact: a nail driven flush, nothing left above the surface. Sha Monk, having no snout, simply covers his face.