Watch the video so that you get the idea of this effect, it is just too great to explain it with words.
You show four cards, actually the Ace to Four to make it easier to follow. Secretly turn over one card and let the spectator guess which one you turned face up. Whatever he answers, you can show in a very fair manner how each card - one by one - turns face up to match whatever he would have said. Finally all cards suddenly turn face up at the same time without any suspicious move!
But just when the spectator believes that the effect is over, you explain the secret of the trick: very special trick cards. When turning over the cards one by one you show that all the standard red backs changed into funny variations, one stronger than the other. Just watch the video.
A great and surprising Finale!