It's hard to describe in text, but I will try. Get a new card without any bends, and place it between your middle and index finger. Bend your wrist inward, and slightly curl your fingers in. Now snap your wrist forward, and as your fingers are pointing directly away from you, loosen up your fingers and let momentum do the rest. Make sure the card is flat, and not bent or else the cards will just flutter away from you. It takes some practice, and it will ruin your cards, so don't use any prized and valuable decks.