Grenade

When fisting your partner, using a tightly balled up fist fully insde them, without warning, you yell BANG!! while simultaneously opening your fist, fully extending your fingers, creating the sensation (and possibly similar physical affect) of a grenade going off inside them.
Warning: a poor manicure may enhance the hamburger effect.

Guy: Last night I was fisting a real dead fish and decided to liven things up with a grenade. She jumped around so much, she almost broke my wrist.