Actually, it's not special to her down tilt. All moves that hit the opponent into the surface they are on with a force less than 2000 have the exact same mechanics. For example, Ness's down air normally always launches with forces above 2000, so you never see it do this, but if you hit a metal character (at low damage), they will get the familiar 'dizzy' animation and have a chance to trip. Of course, if the force you are hit into the ground with is less than 2000, then the ground absorbs the force entirely.
Unfortunately I don't know what kind of frame advantage 'dizzy' offers you, but I do know that you can SDI these dizzying hits.
Unfortunately I don't know what kind of frame advantage 'dizzy' offers you, but I do know that you can SDI these dizzying hits.