During the Southern and Northern Dynasties Period (420-589), there was a Chinese painter called Zhang Sengyao. Once, he visited a temple and painted four dragons on the wall, but he gave none of them eyes. Onlookers thought this odd, and asked why he hadn't painted in the eyes. He answered: "Eyes are crucial for dragons. With the eyes painted in, the dragons would fly away." Nobody believed this, so Zhang Sengyao took up his brush and added eyes to two of the dragons. No sooner had he finished than the two dragons flew into the sky amid a thunderstorm. The two without eyes remained paintings on the wall.
This idiom is used to describe how, when painting, writing or speaking, the addition of just one or two key brushstrokes, sentences, words or phrases could enhance the content.