The magician has a card chosen from the deck (a black 8 or a red 3) and it then replaced and "lost" back into the deck.
The performer tells the helper the card is 18 cards from the top and cuts off that many only to find out it is not the chosen card.
The cards that have been cut off are then shaken and a real 8 ball ( or red 3 ball) falls from the cards. The chosen card is then found to have risen to the top of the deck and the 8 ball ( or red 3 ball) has flattened out into a disc shape.
What you get: