Covering the deck with the hand allows the holder to manipulate the cards so as to view the top card's face (useful in poker or blackjack) or to lift and select a card without the spectator's knowledge (useful in card tricks).
Covering the deck with a hand provides the holder with the ability to view the top card's face (useful in poker or blackjack) or to lift and select a card without the spectator's knowledge (useful in card tricks).