You tap on the cold, hard glass to emphasize the impossible nature of what is going to take place. You lower your hands and seemingly begin to extract one card-- slowly-- from the scatter that the spectator is pressing against on the other side. To the spectators, it actually looks as if the card is sticking half way out of the glass... it takes genuine effort to pull it through.
There is a reason we call it Surgical Card Through Window. It looks like slow surgery on a plain slab of glass. The card is SLOWLY turned over-- it is in fact their signed, selected card on the other side of the window.
Sit back, relax, and enjoy the show. Because when you turn that card over you are going to be watching one.
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