The performer hands half a deck of cards to a spectator and keeps the other half for himself.
He turns his back to the spectator, and asks him to freely pick any card among the cards that he has been given, remember it, and then slide it back anywhere among the magicians cards. Once this is done, the magician turns around again, and without any suspicious moves, names the spectators card!
Easy to do and of great impact on any kind of audience.
The poker size deck takes advantage of a very ingenious system.