As different amounts of light illuminate the wheel, the number of synchronised dancers increase and decrease.
You can find out more about the process here:
This is the wikipedia Zoetrope definition:
A zoetrope is one of several pre-film animation devices that produce the illusion of motion by displaying a sequence of drawings or photographs showing progressive phases of that motion. The name Zoetrope was composed from the Greek root words ζωή zoe, “life” and τρόπος tropos, “turning”.
The zoetrope consists of a cylinder with slits cut vertically in the sides. On the inner surface of the cylinder is a band with images from a set of sequenced pictures. As the cylinder spins, the user looks through the slits at the pictures across. The scanning of the slits keeps the pictures from simply blurring together, and the user sees a rapid succession of images, producing the illusion of motion. From the late 19th century, devices working on similar principles have been developed, named analogously as linear zoetropes and 3D zoetropes, with traditional zoetropes referred to as “cylindrical zoetropes” if distinction is needed.