The illusion comprises a morphing sequence between two faces. The observer has to fixate a dot superimposed on the morph. When the dot is moving, morphing can hardly be seen. However, when the dot suddenly stops, the morphing appears surprisingly strong. Subtle differences in, e.g., the shape of the eyes, the color of the skin, and even gender characteristics are ‘blown-up’ perceptually. Apparently, such differences between faces are easily overlooked when following a moving-dot, but are highly salient when our eyes rest at a single point on the morphing faces.
This illusion by Rob van Lier and Arno Koning is one of the ten finalists in the Best Illusion of the Year 2011 contest. See them all at the contest site. Link