You take a deck of cards and tell the audience that you have made a prediction that is in your inside coat pocket. You proceed to let a spectator choose a card, and then show you have predicted it by removing from your pocket a Jumbo Eight of Spades (for example).
The spectator protests that this is incorrect, so you ask for the name of the card chosen. "The Queen of Hearts" (for example) you are told. "In that case," you say, "I must perform what is called in the trade, a switch." You turn the card around and fixed to the back is a wall switch, you solemnly flip the switch. When you turn the card round again, it has changed to the correct card, the Queen of Hearts.