Effect: Performer shuffles the deck and then riffles through it (see below.) A spectator is asked to say stop before the end of the deck is reached. The performer separates the deck at that point, shows the card to the audience (without looking) and "guesses" the card.
Hold deck horizontally, place thumb on bottom of deck, and middle, ring, and index fingers on top of deck. Place unused hand around deck with thumb on the back of the deck (not the bottom, the back...) Pull top of deck back with top fingers and smoothly lift fingers from top of deck so the cards flip forward one by one.
The trick is in the initial shuffle. When you shuffle, note the bottom card. As they tell you to stop while riffling, separate the cards, but use your thumb to pull the bottom card out, along with the top half of the deck. Hold the chosen half up, facing the audience. Don't look at the card. Tell them the card that you saw when you shuffled. That is their card. When they ask you to do it again, which they will, do it without looking at the deck. That will amaze them.