Fēi yín 飞吟 “Flying Vibrato”

To quote Kwok: “A delicate touch of the string described as ‘like that of a dragonfly upon the water's surface.’ The fingers press the string once or twice lightly. It is sometimes used to stress the end of a note of relatively long duration or the last of two or more notes within a beat.

Ferguson’s Source A differs, saying fēiyín requires a large, not a delicate motion, and does not mention dragonflies, but sources E and F do.

Shuāng fēi yín 双飞吟

Two of the dragonfly-like fēi yín played in sequence. Source F is the only one with this term, so it is likely two quick, light presses of the string.