From Scientific American:
Like tricking a dog into chasing a stick that is not thrown, a stage magician can create the illusion she has tossed a ball into the air when actually she has palmed it. Researchers report that the illusion, which they found could be rather convincing, results simply from watching the magician’s face and not from glancing where the palmed ball would have traveled. “People claim they’re looking at the ball but really they’re making use of social cues,” says a co-author of the report, psychologist and magician Gustav Kuhn of the University of Durham in England.
A magician performing the trick tosses a ball in the air twice and then pantomimes a third throw. “It’s one of these standard tricks in magic. I knew that it was quite powerful,” Kuhn says. To study the source of its power, Kuhn and his colleague Michael Land of the University of Sussex in Brighton, England, videotaped Kuhn doing the trick in two ways: on the final fake throw, he would either look up where the ball should have flown or he would look down at his hand.