While Absolute Cleavage is about revealing most of the breast from the center, and Underboobs is about doing the same thing from below, this is about revealing them from above. In other words, an off the shoulder neckline that looks like one deep breath will cause the dress to fall completely off.
The actual outfit doesn't matter. It could be a bathing suit, a catsuit, a mini-dress, a leotard, or even a formal gown. In real life, this sort of thing usually involves either a lot of adhesive or hidden fasteners.
Compare to Magic Skirt.