This is when a girl thinks she is being nice by giving you a hand job, but actually hurts you by placing her fingers up and down on the head of the penis causing extreme irritation.
Joey: Lets go into the bathroom.
Rachael: Ok Joey.
Gets intimate(in a bathroom)
Joey: AHhhHHH, what the fuck are you doing? I'm leaving.
Michael: So how was rachael after all?
Joey: It hurt like hell.
Michael: Please tell me she did not clamshell you...
Joey: She did, oh she did.