Card holing in blackjack involves being able to see the hole card or the downwards facing dealer card. This happens thanks to a sloppy dealer who's not taking enough care to keep the down card hidden. For the player it can be very helpful in making decisions as to whether to bet high or low. The problem comes from the fact that you'd have to wager counter to perfect strategy. This will draw attention, as no one will understand why you made that particular game play. You may be asked to leave.