Take a deck of cards from your pocket. 
Call attention to the folded card on top of the case, held in place by an elastic band. 
Remove the deck from the case, and hand it out for shuffling and examination. It's a normal deck. 
Invite a spectator to take any card from the deck and sign the face. 
The card is lost in the deck. 
Ask your audience how long they think it will take you to find the card. 
Explain that to make the challenge even more difficult, you'll put the cards behind your back. 
Finding a card without being able to see the deck would be impossible, no? 
Your spectators give you 10 seconds, say. You bring the deck behind your back and come out immediately with a card in hand. It's the wrong one! You try again! Still the wrong card! You have clearly not risen to the challenge. 

BUT, are you sure? Take the card that has been folded on the card case since the beginning of the trick at the tips of the fingers. 
No, it's not possible, your spectators will say to themselves. The card has been there since the beginning and no one has touched it. 
You hand the card to the spectator and invite him to unfold it. 
I know it's completely impossible, but the card he is now holding is his signed, selected card! It's HIS signature, it's HIS card! Absolutely NO DOUBT! 

Note the following: 

- NO manipulation! 
- As easy to do as it is incredible! 
- The box does EVERYTHING. Nothing to do! The card changes instantly, just as you approach it! 
- Resets in less than 20 seconds! 
- The subtle mechanics and precise manufacture take care of everything! 

Each box is handmade to fine tolerances. Your hand approaches the card and the card is immediately and automatically switched for another. As simple as that!