The spectator names any card out loud.
The magician draws an x on the cellophane of the card box and makes it visually disappear.
The magician pulls the card out of the box and shows that the drawn x has moved on the card that was freely named by the spectator.
But that's not all!
The magician takes the card with and X on it, hits the card box and the X jumps again on to the cellophane of the box, leaving the card unmarked, which can be given to the spectator!