You let someone pick a card from the deck, he/ she thinks they chose the card randomly out of the deck. In your mind, you've already know what card they picked up from the deck. That is a forcing a card..

Forcing a card on someone means, before the trick even starts, you already know what card they are going to pick. Instead of letting the audience pick a card randomly from the deck, you let them pick what YOU want them to pick. Forcing a card is not that simple. It should allow the audience to think that they are REALLY choosing a card out of 52, and yet letting them pick the card of your wish.

There are many ways of forcing a card on someone, what featured here are 2 of the simplier ones, but still able to deliver the desired results.