Spinning Dancer - Wikipedia
Bistable perception
Depending on the perception of the observer, the apparent direction of spin may change any number of times, a typical feature of so-called bistable percepts such as the Necker cube which may be perceived from time to time as seen from above or below. These alternations are spontaneous and may randomly occur without any change in the stimulus or intention by the observer. However some observers may have difficulty perceiving a change in motion at all.One way of changing the direction perceived is to use averted vision and mentally look for an arm going behind instead of in front, then carefully move the eyes back. Some may perceive a change in direction more easily by narrowing visual focus to a specific region of the image, such as the spinning foot or the shadow below the dancer and gradually looking upwards. One can also try to tilt one's head to perceive a change in direction.
Another way is to watch the base shadow foot, and perceive it as the toes always pointing away from you and it can help with direction change. You can also close your eyes and try and envision the dancer going in a direction then reopen them and the dancer should change directions. Still another way is to wait for the dancer's legs to cross in the projection and then try to perceive a change in the direction in what follows.
You could also try using your peripheral vision to distract the dominant part of the brain, slowly look away from the ballerina and you may begin to see it spin in the other direction. Perhaps the easiest method is to blink rapidly (slightly varying the rate if necessary) until consecutive images are going in the 'new' direction. Then open your eyes and the new rotational direction is maintained. It is even possible to see the illusion in a way that the dancer is not spinning at all, but simply rotating back and forth 180 degrees.
Slightly altered versions of the image exist in which an additional visual cue facilitates the perception of counterclockwise spin and clockwise spin. Looking at one of these can then make the original dancer image above spin in the corresponding direction.
