Equivocation/Equivoque/Magician's Choice

Wikipedia: Equivocation (magic)

Equivocation (or the magician's choice) is a verbal technique by which a 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.[1] For example, the performer may deal two cards to the table and ask a spectator to select one: if the spectator chooses the card on the left, the performer will say something like "you keep this card, I'll take the remaining card". If the spectator chooses the card on the right, the performer will take that card. Thus, the choice of which card to use is really made by the magician.

The effectiveness of the equivocation involves the "information gap" between what the spectator knows and what the spectator thinks they know.

Equivocation tends to lose its effectiveness if repeated in the same context, since the spectator gains more information from one performance to the next, thereby shrinking the information gap. For example, a spectator may wonder why their choice was kept in some cases and discarded in others.

Equivocation is a particular form of alternate ending forces where double entendre wording is used and a different pattern of results to questions can be noticed, but its real strength is best realized when augmented with artful psychological techniques.

See:

E'voque: Docc Hilford

Tricks using Equivocation:
-Proof Positive
-Max Maven Routine