When you slide your dick in someones ass while they have diarrhea. The pressure causes them to release their bowels on your dick. Then you thrash harder causing the feces to splatter on their back. Thus being a back banger.
A backseat banger is someone who was invited to a threesome but doesnt do anything but sit in a corner to tell the other guy what to do, similar to a backseat driver.
Girl: should we invite Jeff to our house for a threesome?
Guy: no, I've heard that he is such a "backseat banger" ong