The Riffle Force
The magician riffles through the deck and asks the spectator when to stop, at which point the magician shows the spectator the card he/she stopped at. The card the spectator sees is actually the original bottom card of the deck.
How It's Done
Memorize the bottom card of the deck, next hold the deck up face down with your left hand, thumb on the left and your other fingers on the right. With the index and middle finger of your right hand, start to riffle through the cards, and as you do so, use the thumb of your right hand to slide the bottom card back, this is hidden from the spectator by the deck and your right hand. When the spectator says stop, quickly pull back the cards you have riffled through with your right hand, and put pressure against the bottom card with your right thumb so that the bottom card comes with the stack as well. Hold up the riffled stack to the spectator to show the selected card, which is now the original bottom card of the deck.