Wildereachday got it right. If you haven't read his answer, go back and read it.
This is the premise to an amazing prediction/ mentalism trick that I do (taught to me by Mr. Chang). Very hard hitting.
I will give you the effect, you guys try to work out the method =)
A deck is handed to a spectator and the spectator is asked to take out a handful of cards, any amount between maybe 15 and 20 cards (this is just an average amount). The rest of the deck is discarded and not looked at. The spectators pile of cards is shuffled and the spectator is asked to deal out a certain number of cards and give them to the magician to keep as his pile. Amazingly, the amount of red cards the magician holds is the exact same number of black cards that the spectator holds, matching the magician's prediction. The spectator is reminded that he did all of the shuffling and dealing, and that he chose the original pile of cards.
Great trick =)