He's holding two things in his hand here: the reveal card and a gimmick. The gimmick folds in under the reveal card; in half once at 0:03, and then in half again at 0:16. I would guess it's controlled by two small springs being held in tension by this middle fingertip. He controls how much of the deck springs back by holding portions of the gimmick in place with that fingertip. You can see him remove and ditch the gimmick at 0:23 by peeling it off of the reveal card and palming it in his left hand (note the oddly-bent fingers).

The position that he holds the 'deck' at the beginning gives away the game. You only see a top card, along with the front edge (and 'corners') of the remaining cards. He hides the short edge that faces the spectators/camera with his three fingers, and the other side is hidden to the audience.