The magician reveals the deck switch secret to the spectator.
He tells the spectator that magicians can switch decks faster than the eye can see. Because the back color of the decks is the same, the switch is indiscernible. To illustrate this fact the magician says that he will switch a blue deck for a red one. He throws the blue deck from his right hand to the left, but the moment of the switch is indiscernible too. The only thing that the spectator realizes is that the thrown blue deck changed to a red one.
Then the magician takes out the red deck from its red box and asks the spectator to choose a card. The chosen card is placed on the table. The magician throws the rest of the red deck from one hand to the other and the red deck becomes blue. The only red card is the chosen card. Finally the chosen card is placed on top of the deck and the deck is thrown from one hand to other and the deck become red again.