This effect was just previewed at the 2017 Magic Live convention in Las Vegas and sold out completely in the first two days. Magicians who took time to learn the effect while there said it was the best purchase they have made in a decade.
23 allows you to dramatically reveal the card that a spectator has only thought of. No forces, no formulas, no math, no questions. The result is dramatic and unforgettable, and the method is as close to self-working as you can imagine.
Here is how it looks: People say that numbers influence us: 7 is lucky, 13 is unlucky, and there's a powerful superstition surrounding the number 23. It is believed that actively trying to avoid the number makes the connection more powerful. This is known as the 23 Enigma.
Your spectator is invited to cut off less than half the deck and to shuffle the cut-off cards. He is told to fan the cards in front of himself and mentally select a lucky card from all the cards he sees. The spectator turns the fan of cards face down again and shuffles them so even he has no idea which card is the one he mentally selected.
The spectator intentionally tries to upset the 23 Enigma by silently removing a group of cards and hiding them in the card box. At no time does the performer force, influence, or know which card the spectator is thinking of or how many cards have been hidden.
The performer suggests that the Enigma occurs no matter what you do to try to avoid it. After fairly and openly counting down exactly 22 cards in the pack, the performer reveals that the card in the 23rd position is the only red-backed card in the deck and has a giant number 23 marked on its back.
Without ever speaking the name of his selection, the spectator turns over the red-backed card to discover that it is exactly the same as his mentally selected card.