Imagine having a normal deck of cards and a coin. Your spectator picks any card from the deck, signs it, it's put back and lost in the deck. The deck is put into the box.
You show a coin to the spectator, put it on the card box and make it vanish. Then you open the box and your spectator sees that the coin is now located in the middle of the deck. You cut the deck at that position and it's exactly spectator's signed card!