[1] The performer exploits the ambiguous wording of their patter to win the bet in a manner unexpected by the audience.
[1] The performer claims that they will be able to find a spectator's card in the deck by some means such as touch alone.
[2] The performer will at this point offer to wager a sum of money that the next card they turn over will indeed be the spectator's.
[1][2] The magician will have forced the spectator's card in some manner, ensuring they knew its position in advance or could spot it later.
[2] One method is for the performer to note the bottom card of the deck prior to starting the trick.