Summer Cummings' tits kind of do that when she's riding. I'm thinking of a scene from the movie Boobsville Sex Academy where Tanya Danielle is wearing a strap-on and Summer is riding her. Her boobs bounce around a little but stay pretty much the same shape throughout. Not jiggly like natural ones would be. Some dildo tit-fucking in there, too, which makes Summer's tits look even firmer. Pretty hot clip, though the movie itself wasn't the best.