The magician folds two cards in half -- one lengthwise and the other counter-lengthwise. The long card is passed through the short card, and when it emerges it has turned inside out. The magician continues pushing the card until half of it has emerged, then rips the card in half and shows that this act of unpremeditated violence has stopped the inversion process, leaving half the card inside out and the other half outside in.
Card Warp makes use of a small, easily hidden gimmick called the Walton Card Inverter. The inverter is a piece of plastic about the width of a card folded lengthwise. Made of transparent plastic, it looks a bit like a compact Klein bottle that has been pressed in a book.
When a card is passed through the Inverter, one side goes up one ramp and the other side goes down another, effectively turning the card inside out. Pulling the card back through the inverter reverses the process.
To perform the trick, the magician merely hides the Inverter in a card during the performance. The rest is pretty automatic. The only caveat to that statement is that the magician must be very, very careful not to accidentally stick a finger into the Inverter. Doing so can have gruesome consequences, and even some large hospitals do not have staff trained in the repair of inverted fingers.