The pre-masturbatory act of grabbing your flaccid penis with your fingers or hand, and flapping or bouncing it around to predicate an erection.
"Yo, you try this Flappy Bird game yet? I heard it's real addicting."
"Heh, I'm playing flappy bird right now."
"Well how is it- what in the hell man, you're disgusting!"
"Well, playing flappy bird is more of a gateway deal, it only gets addicting once you're actually jerking off. "