Pluck

Southern action where a middle finger nail is pressed firmly against the thumb on the same hand, the middle finger nail is released (flicked) from the thumb in a backwards quick motion to strike a person or thing as a joke, to get someone's attention, or to annoy. Sometimes, but very seldom called a thump.

Pluck is Snapping a finger to a song but the middle finger nail's exertion and direction is completely opposite when leaving thumb.