chunnel
When two people place a well lubricated tube (either with an artificial lubricant, or something natural such as urine or jizz) between themselves, and place an end into each of their assholes. After the tube is in place, one person poops into the tube with great force, propelling the feces through the tube into the other persons asshole. Following the penetration of the feces, the reciever goes through a pooping motion to send the feces back to the original owner. 

Sometimes the second person will add their own feces to the original feces, resulting in a larger poop, which is rumored to add more fun.

The poop can sometimes be lodged in one persons anus, causing the sex act to end until the poop is dislodged, or an enima can remove it.

Usually practiced between gay males.
Bernie and Robert chunnel-ed back and forth for hours, and by the end of the night, their feces had formed into bullet like objects that flew back and forth through the chunnel pipe with great speed, due to the contriction of their assholes, and the tube lubrication.