A classic gag! You've forced the Eight of Hearts or Spades, after some banter, you then open your jacket and slowly pull out a card, stopping after 3 pips are showing so that the spectator sees the Three of Hearts or Spades and ask if this is the correct card. They'll obviously say no, so you then pull it out a little further asking if it's the four, or the five and so on until you eventually pull out the entire long card to reveal that it is in fact the correct card. A very funny routine.
The card is double sided, spades on one side and hearts on the other.