David Blaine asks a spectator to sign a card and replace it in the deck. He also asks the spectator to shuffle the cards. Blaine then throws the cards at a bottle positioned behind the spectators and incredibly, the card which was signed can be seen in the bottle.
Part of the secret to this trick is that you need to show the spectators a different trick before this one in which the same spectator signs a card of the same number and suit that is forced when showing the card in the bottle trick a while after.
You can then keep the card and perform another trick not involving cards as a distraction and secretly slip the card to an accomplice who can slip it into the bottle whilst your spectators are being entertained and distracted by you!
Then when you go on to perform the card in a bottle trick, you simply need to force the same card as the one secreted in the bottle and when you throw the deck of cards at the bottle, you then of course tell your spectators to look at it and at that point you slip the newly signed card, which you have retained, into your pocket.