画huà龙lóng点diǎn睛jīng

Literal: paint a dragon (and) dot in the pupils
Figurative: To add the one crucial finishing touch that brings a whole work or speech to life.

Where does 画龙点睛 come from?

According to a Tang-dynasty record of painting, the painter Zhang Sengyou painted four dragons on a temple wall but left them without pupils, saying that if he added them the dragons would fly away. People thought it was a boast and pressed him. When he dotted in the pupils of two of them, thunder broke out, the two dragons shattered the wall and rose into the sky; the other two, still eyeless, remained on the wall.

Source (出处): 《历代名画记》

How is 画龙点睛 used? Two examples

文wén章zhāng本běn来lái很hěn平píng淡dàn，结jié尾wěi那nà句jù话huà真zhēn是shì画huà龙lóng点diǎn睛jīng。
The article was rather flat, but that closing line was exactly the masterstroke that lifted the whole thing.
设shè计jì稿gǎo加jiā上shàng这zhè个ge小xiǎo图tú标biāo，立lì刻kè有yǒu了le画huà龙lóng点diǎn睛jīng的de效xiào果guǒ。
Just by adding that little icon, the design instantly gained that finishing touch that brings it to life.

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