They won't see this one coming!
This is a specialty card that allows a surprise ending to a trick- you can produce a 14 of Spades card. For example, let the spectator choose a card from your deck. You "bet" that the card you are holding is exactly twice the value of the spectator's card. The spectator happily accepts the bet, as he is holding the 7 of Spades. Lo and behold- your card is the 14 of Spades, exactly twice his card's value! For this idea, you would need to force the 7 of Spades card. See the second video below to learn how to force the card.