When the hills had been opened and the floods governed and peace had come, it drifted away to the Eastern Ocean to hold the sea-palace steady.

開山治水 is the standard four-character summary of what Yu did — cut through the mountains, govern the waters. Once the job was done the rod had no work, and 流落, to drift and come to rest, is the word for something that ends up somewhere by no plan of its own. 鎮 is to hold down or pacify by sheer weight: it became a paperweight for the dragon king's palace.

As the days lengthened into years it gave off coloured light; it could shrink, could grow, could shine clean.

Three 能 in a row, could and could and could, which is how the line does an inventory of powers without listing them. 消 and 長 — shrinking and growing — are the pair the chapter will use again a few pages later for what these weapons do in a fight.

Old Sun had the lot to fetch it away; it changes without limit at a word of the formula.

有分 is a fixed expression meaning to be fated or entitled to something — a share allotted to you — and 分 is read fèn in it. It occurs across eighteen chapters of this novel, always in that sense. Wukong is not claiming he took the rod; he is claiming it was his to take. 口訣 is the spoken formula, the words that make a technique work.

Want it large and it fills the whole space between heaven and earth; want it small and it is like the joint of a needle.

The line that gives the rod its character, built on one plain contrast. 彌於宇宙間 is to fill the entire span of space and time — 宇宙 means both. Against it, 針兒節, a needle's joint, which is what came out of his ear a moment ago. The 兒 counts as a full syllable here: the line has seven, and it needs all seven.