Now ask the spectator to choose a card from the deck and while he is doing so you quickly turn the deck upside down. Since there is a double-sided card so the spectator will not be able to notice.
Once the spectator has chosen and viewed the card you it back inside the deck. Although the spectator has inserted the card with its face down but with double sided card on top it will be face up in the deck.
Finally you will bring the deck to the normal position with the double-sided card going down again. As you show the cards to the spectator the one he selected will be appearing upside down in the deck.