Explanation:

You setup the deck with all face down except for one card at the bottom. You spread the card asking your spectator to choose one card. Be careful not to reveal the face up card at the bottom. You tell your spectator to pick the card, look at it, and remember the card but as he does, you flip the cards over. So you have one card face down and the rest face up. You tell your spectator to put the cards somewhere in the middle of the deck. Be careful not to reveal all face up cards. Then tell your spectator that you will put all cards behind you and reveal your card within a split second. What will you do here is you just get the top card and reveal it. This is obviously not his card. You then spread the cards face up to reveal one card that is face down. This is your spectator’s card.

by : Enrico Lorenzo, source : http://card-magic.blogspot.com