When a person doesn't shit for days and takes a bunch of laxitives an hour before anal intercourse. When the giver is about to climax, the receiver, having held it in for the entirety of the sexual act finally releases their bowels all over the penis of the giver. The pumping action acts as a suction pumping all the shit out of their ass not unlike an oil pump on a rig. Usually done as a retaliation to unwanted anal entry.
When Sam gave Mary anal, she retaliated with the Pensacola Pumper.