First, the magician performs the trick. The spectator selects a card, places it into the deck, and the magician finds it.
Then, the magician explains the secret - he had previously separated the reds from the blacks, and made sure the spectator placed their card in the "wrong" section. This guaranteed that the magician would find the card (the only red card amongst the black cards, or vice versa).
The magician performs the trick again, this time explaining the secret. Importantly, he states that the trick does not work if the deck is shuffled - this destroys the "set up" of all blacks and all reds, making it impossible to find the chosen card. He then SHUFFLES the cards! There's NO WAY the card can now be found. Still, he finds it because "the set up" has magically been RESTORED - all blacks and all reds are once again together, with the spectator's card amongst the "wrong" color!
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