Put three cards face-up on the table. Ask a spectator to choose one mentally and remember its position.

Flip the three cards over. Turn around and instruct the spectator to show the other spectators which card he has chosen, then have him switch the two other cards behind your back.

Now face the table again and instruct the spectators to switch cards as many times as they wish in front of your eyes.

Reveal the card originally chosen by the spectator.

Please explain how does it work.