The act of stretching one's scrotal skin out with two hands - using thumb and forefinger - parallel to a primary surface (i.e. a bed or floor) until smooth, with the penis resting on top, utilizing the grip and taut skin to move the scrotum up and down at various speeds to have the penis bounce off of it continuously.
tram·po·line Verb /ˈtrampəˌlēn/
trampolined (past tense); trampolined (past participle); trampolines (3rd person singular present); trampolining (present participle); "Trampoline Dick"; "Trampoline Cock"; "The Trampoline"; "The Bounce"
"I ran out of tricks once I actually got her in bed, so I just showed her The Trampoline and bounced my way back into her heart and right between her legs."
"Nah man! she would not let me but she did let me trampoline her..."