A spectator chooses a card and holds it against the magician's stomach and thinks about the card. The magician looks like he is in pain, and grasps the spectator's wrist and asks them to move the card away. The performer lifts up his shirt to show his belly which has a picture of a card in red on it, the spectator's card!
This is a very simple but very effective trick, as performed by David Blaine.
