Occurs when the tail-end of a substantial turd (i.e. a turd the size of a zucchini or larger) whips the back of your balls while you're sitting on the can.
Generally, the physics of a shit whip require that the head of the turd first makes contact with the bottom of the bowl, thus providing the turd with the necessary leverage to allow for a slow, frontward collapse. If the cosmos are aligned just right, the motion will carry the tail-end of the turd forward and it will whip the back of your balls on the way down.
"Dude, I just got shit whipped."