This trick requires a gaff card known as a double-backed card. Quite simply, both the front and the back are backs of a deck of cards. To start this trick, place the double-backed card on top of the deck and put the deck in its box.
Remove the pack from the box and spread the cards, face down, remarking that they are all face down and none are face up.
Square the deck and deal the top card, the double-backed one, to the spectator, asking him to simply hold it in his hand.
Turn the deck over, so you are holding a face-up deck while he holds a face-down card.
Instruct him to place his card anywhere into the face-up deck. When he does so, square the deck up once more.
Explain that cards can be stubborn and that they need some discipline. To enforce this discipline, riffle through the deck while telling that card to turn over or else it will be punished.
Turn the deck back over and go through each card to show that they are all face down. Since he placed a double-backed card into the face-up deck, both sides are face down, so to speak.