the ol bait n switch
One has to sit on a tree branch about 11 1/2 feet high. Any higher is dangerous, and any lower will result in a failure of the deed. Once one pulls their pants down, and is ready for the act one has to proceed to poop. Immediately, without hesitation, one has to fall off the back end of the branch, backflipping, and in turn landing on the ground flat on their back. At which point the poop impacts with one's chest. This form of self-masturbation takes many years of experience and should not be perfomed by an amateur. The branch can be substituted with any form of a bar up in the air.
Johnny: How'd you break your neck?

Fred: I did the ol bait n switch from a branch that was too high!

Johnny: Haha, they warn you shouldn't do that!