In the Southern and Northern Dynasties Period (420-589), there was a painter called Zhang Sengyou. Once he visited a temple and painted on the wall four dragons, but gave none of them eyes. The onlookers felt that this was odd, and asked why he hadn't painted the eyes. He answered, 'Eyes are crucial for dragons. With the eyes painted on, the dragons would fly away.' Nobody believed this, so Zhang Sengyou 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 stayed painted on the wall.
This idiom is used to describe how, when writing or speaking, one or two key sentences will enhance the contents.