Double penetration sex, with one man fucking the girl in her vagina in and the other in the ass. They alternate thrusts to create a piston action, much like that of an engine.
"You take her in the ass, I'll take her in the pussy and we'll piston her"
by Color Rot January 06, 2009
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A Sexual act similar to cave diving. The male not only sticks his head in the woman's vagina, but proceeds to climb entirely inside of her. Once inside of her he sticks his penis out of her vagina. Using his penis the girl goes and engages in intercourse with someone else. (The only downfall to this act is that a cesarean section will need to be performed in order for the guy to come out.)
Friend 1: Did you hear about Jessica?

Friend 2:No what happened?

Friend1: Her and Jack did the piston!
by The Pistonator September 03, 2014
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