The magician approaches a passer-by and asks that they hold a pack of cards and to merely think of any card that is in the pack.
"Are you thinking of one ?" - "Not the Ace of Spades ! " - "Think of another ."
"OK - got one ? - Yes ? - OK, I want you to visualise your card turning around, can you do that ?"
"What's the name of your card ?"
The spectators announces his 'Thought card' - "The 5 of Diamonds".
The magician takes the pack from the spectator and removes the deck - fanning the cards face up he comes to one face down card.
"What was your thought card ?" asks the magician again.
"The 5 of Diamonds" replies the spectator.
The spectator takes the face down card and turns it over - it is the 5 of Diamonds !!
A special deck of cards known as an Invisible Deck.
In this deck the cards all have 'roughing fluid' applied to their backs. There are various set ups but the cards are generally arrange in 'pairs'
eg; back to back, face-up face-down, red to black and odd to even, the value of the pairs making a total of 13.
There is more to it than this but this gives you the gist of this gaffed deck.
What the magician must do is to memorise the combinations and pairs so that he is able to locate the 'pair' of the 'Thought card'. He must also remove the deck from the box so that the 'thought card' is face down (half the deck will also be face down with their paired cards).
Sounds complicated but it is relatively easy once learned.
As the cards are being fanned a squeezing pressure must be applied and maintained to the deck so that the back to back 'pairs' do not separate.
Once the 'pair' to the thought card is located the pressure is released and the face down 'Thought card' is pushed forward.