Method
The principle behind the trick is simple: the deck in use is either stacked or partially stacked prior to performance, or manipulated in the course of performance to form a partial or full stack, in which there is a separation between the red and black cards, with a run of each colour comprising either half the deck or a substantial portion of the stack near the top.
In the first round of dealing face-down cards, all of the cards dealt will be of one colour. The switching of sides occurs at the halfway point through the deck because that is the point at which the first colour cards will be exhausted and the second colour of cards will begin to be dealt.
To understand why this enables the trick to work, think about what will be in the two lines of cards at the end of the trick. The left-hand line will begin with (for example) a black marker card (placed at the start), followed by the face-down black cards from the top of the deck, then the red marker card (placed at the side switch), then the face-down red cards from the bottom of the deck. That's (black marker) (black cards) (red marker) (red cards). In other words, this line is exactly right, and is given to the subject to gather up.
The right-hand line will begin with the red marker card (placed at the start), followed by the face-down black cards from the top of the deck, then the black marker card (placed at the side switch), then the face-down red cards from the bottom of the deck. That's (red marker) (black cards) (black marker) (red cards). Although incorrect, this can be instantly made to appear correct by moving the red marker card from one end of the line to the other, or by reversing the order of all other cards in the line. This must be performed by the magician during the act of gathering up the right-hand line of cards, while the subject is distracted by gathering up their own line. Since the cards are gathered into a stack at that point, this is straightforward.
Read more about this topic: Out Of This World (card Trick)
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