The magician shows a deck that has been cut in half diagonally.
The cards for both halves are fanned face up to show they are mixed then turned back face down.
A spectator then each select one card from the first half of the deck,
and a second spectator selects a card from the second half of the deck.
Amazingly, when the selected halves are put together and turned face up they match to form a complete card!
A truly baffling effect!

Split Deck Bicycle