1. When a man or a chick with a strap-on is fucking a man or tranny in the ass from behind (doggy style).
2. The person in the aft position grabs the other persons dick, pulls it back to where it is between the legs of the two people fucking.
3. Starts to jerk it off in the open space available. Being that the only available space in this fucking condition is to either side it is called Side Piping.
Tranny to tranny friends - That dude bent my dick around behind my leg and jerked it off while he nailed my ass. He made me cum but I think we should charge extra for Side Piping!