Imagine a spectator freely selecting a card from a normal shuffled deck and signing its face. The performer then shuffles the signed card back into the deck and returns the deck to its box.
Then the performer slides two face-up jokers halfway into the card box on top of the face-down deck (the jokers can even be paper-clipped together). The two out-jogged jokers visibly penetrate down through the encased cards to the bottom of the deck and then visibly rise back up through the cards to the top of the deck. The performer dumps out the deck and spreads the cards. A single face-down card is sandwiched between the two jokers; it’s the spectator’s signed card!