Bicycle Brainwave Deck - Gaff Deck
The magician places a deck of cards on the table locked into the case. A spectator thinks freely of a card and names it. The magician takes the deck opens it with his hands and shows how a single card is turned face up. Clearly it is the named card from the spectator, which will also be the only one with the back different from the others.
? An effect of big impact on the public because the card is freely thought. The spectator can name any card in the deck.