When a man drops his trousers, or pulls his penis out, and gyrates around, so that the penis flops from side to side, or up and down, in motion. Usually done as a sign of extreme joy, or excitement.
Also a form of erotic dance, by strippers.
This is where the stripper will thrust their pelvic forwards, after stripping naked, and make their penis twirl or flop from side to side, while moving their bodies in a sensual slinky type manner.
Not as easy to do, especially if one is endowed with an average sized penis.
Many erotic dances, contain some form of the 'willy dance', depending too on the penis size, when semi erect.
Fully erect penises, are difficult to make flop around, or slap the thigh.
Generally a soft or non erect state is preferred when performing this form of expressing one's excitement, and/or arousal.