The magician shows two spectators an opened red deck of cards and a brand-new blue one, which is still sealed with cellophane. They are like twins! Only their color is different.
One spectator selects a card from the red deck. The second spectator holds the new, sealed blue deck in their hands and covers the box so the magician can't touch it. With a wave of the magician's hand, the selected card's twin (same value, same suit) now appears on the outside of the blue deck, under the still-sealed original cellophane!
The miracle happens in the spectator's own hands!