When a male engaged in vigorous anal sex breaks his dick inside said anus. Upon the breaking (and after all the anal pressure) the person receiving the anal sex becomes incapable of containing themselves and releases their fecal matter all over the broken penis. Therefore, making it a "Rusty Turn Boner."
Todd: Who was on the phone?
Chris: That was the hospital, Hank got admitted...
Todd: Oh my God, what for?
Chris: They say he got a Rusty Turn Boner and is going to need surgery.