When a character or model poses with their back to the audience, but twisted round to look at the viewers so that their torso is almost pointing backwards. If the fanservice is more blatant, they'll probably be sticking their butt out and curving their spine vertically rather than merely twisting sideways. Probably used for a few reasons: Popular with glamor models and anime characters. Naturally characters not portrayed by real life actors can pull this off more effectively, since they can twist in ways that would snap a real person's spine or at least be darn uncomfortable.