(With either male or female partner - U.S.S.R can only be done by a gay couple).
The guy lays down and the other partner squats as if they're going to ride him cowboy style.
As the top partner comes down on the dick for anal, that person starts taking a dump so that the shit comes out and is then pushed back in by the dick they're sitting on.
Also see U.S.S.R for the Ultimate Soft Serve Reverse.
Yeah, I gave her a SSR, mmm tasty.
Gotta love the soft serve reverse.