The magician holds a card in his hand that is not the spectator's selected card. With a quick snap of the fingers the card turns into the selected card.
How It's Done
Get the spectator's card to the top of the deck. Perform the Double Lift and ask the spectator if the card (which was the second to top card) is theirs, they will say "no". Hold the two cards in your right hand with your thumb on the back and your pointer, middle, and ring finger on the front. Now take your ring and pointer finger and snap the card visible to the spectator behind the spectator's card. The snapped card will be hidden behind the spectator's card, held by your middle and ring finger against the bottom of your thumb. Also note while do do the snap the two cards will make a noise as they pass by each other, so it is best to use your left hand to somehow flick the cards as you do the trick to hide the source of the noise.