点睛之笔

diǎn jīng zhī bǐ

the brush stroke that dots in the eyes (idiom); fig. to add the vital finishing touch

the crucial point that brings the subject to life; a few words to clinch the point

点

diǎn

to touch briefly; to tap

睛

jīng

eye; eyeball

之

zhī

(possessive particle, literary equivalent of 的[de5])

笔

bǐ

writing brush; writing implement (pen, pencil etc) (CL:支[zhi1],枝[zhi1])