How can a magician make you 'freely' pick the card he wants?

A force, in stage magic, is a method of controlling a choice that a spectator makes during a trick. Some forces use sleight of hand: a spectator appears to select a random card from a deck, but is instead handed a known card by the magician. Others do the work with words.

That second kind is called equivocation, or the magician's choice. It is a verbal technique in which the magician gives an audience member an apparently free choice but frames the next stage of the trick in such a way that each choice has the same end result. Here is a standard example, described without any trick of the hands. A performer deals two cards on a table and asks you to select one. If you choose the card on the left, he hands it to you. If you pick the card on the right, he takes it as his own and gives you the other. Either way, you end up with the card he intended.

Why does it work? The effectiveness of equivocation involves an information gap between what the spectator knows and what the spectator thinks he knows. You know nothing about what will happen to the two cards you first select, yet you think you are making a free choice in an otherwise scripted sequence of moves.

That is also why equivocation tends to lose its effectiveness if repeated in the same context. The spectator gains more information from one performance to the next, which shrinks the gap, and may wonder why his choice was kept in some cases and discarded in others. One particularly widely used form of equivocation uses books, and is known by its own name, the book test.