David Blaine takes two packets of ten cards -- ten for himself and ten for a spectator. He asks the spectator to look at one packet and just think of a single card. Blaine then asks the spectator to count the cards, and it is revealed that there are now only nine. Blaine shows that his packet now has eleven, and one of them is the spectator's thought-of card.
At one point, after Blaine asks the spectator to think of a card, he "helps" the spectator with squaring up the packet of cards. Blaine takes this opportunity to secretly help himself (by means of a beside steal, used when you're beside someone who has a deck of cards) to the thought-of card which is secretly transferred to Blaine's deck.
Some may wonder how Blaine knows which card the spectator thought of. Simply put, everyone chooses the three of clubs.