Secret: You have secretly reversed the top card so the deck looks as if its face-down when its really face-up. When the spectator places his or her card back into the deck, the selected card is reversed and facing the other way.
When noone is around, prepare the deck by reversing the bottom card so its facing the other way. Note that the picture above is strictly for explanation. As you perform the trick, the spectator should never notice that the bottom card is reversed.