A sexual position where two females lay on each other back to back. One famale is on all fours while the other lays back to back on top of her. The male can finger one and use his dick on one, rotate using his dick, eat both out, or eat one and finger one.
Guy: Dude i hung with these smokin hot chicks last night and eventually convinced them to let me get em in a back stack.
Guy's friend: Nice dude! Lemme know next time and maybe we can tag team.