A sporting proposition

You can use any deck for this trick, and the spectator can shuffle it.

When you take the deck back, you must memorize the bottom card : it will be your key-card, without making it obvious.

Now allow the spectator to take the top half of the deck, shuffle it, and remember the top card. They can put it back on top of the deck.

Next, you can riffle shuffle the deck, maintaining the top and bottom cards, and cut it.

Now deal cards face-up on the table. The next card after your key-card will be the spectator's selected card.

To fool the spectator, you can deal a few cards after their card, and bet that the next card you flip is their's (hence the trick name). Now go back to their card, show it, and flip it.