We know from experience that rigid objects can rotate and move but not stretch. A human head is considered to be a rigid object, and therefore is not expected to spontaneously change its shape. This expectation allows us to create the illusion of a head rotating around a vertical axis.
By dividing the 3 dimensional model of a head into left and right hemisphere components, a rotation effect can be achieved by stretching one side while compressing the other along the horizontal dimension via computer algorithms.
The present illusion reveals an interaction between cognitive assumptions about rigidity and visual perception.