入rù木mù三sān分fēn

Literal: To enter the wood three fen (to sink three tenths of an inch into the board).
Figurative: Of an analysis, critique or description: penetrating and incisive, getting to the very heart of the matter. (Originally, of forceful calligraphy.)

Where does 入木三分 come from?

It is said that the great Jin-dynasty calligrapher Wang Xizhi wrote some characters on a wooden board that were later to be carved. When the craftsman began scraping the wood to engrave them, he found that the ink of the brush had soaked into the board to a depth of three fen. Hence the expression first praises the force of the brushstroke and, by extension, anything sharp and penetrating.

Source (出处): 《书断》

How is 入木三分 used? Two examples

这zhè篇piān评píng论lùn分fēn析xī得de入rù木mù三sān分fēn，把bǎ问wèn题tí的de根gēn源yuán说shuō得de一yī清qīng二èr楚chǔ。
This commentary analyzes the matter incisively and lays out the root of the problem with perfect clarity.
他tā对duì人rén性xìng的de刻kè画huà入rù木mù三sān分fēn，让ràng读dú者zhě读dú完wán久jiǔ久jiǔ不bù能néng平píng静jìng。
His portrayal of human nature is penetrating and acute, leaving the reader unsettled for a long while after finishing.

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