The two red ellipses are actually identical and oscillate synchronously. Two perceptual illusions occur, which are triggered by the four small black spots.
The black spots are stuck on the red ellipsis on pendulum 1 and emphasise the common rigid body movement. However, in pendulum 2 the black spots move relative to the swinging ellipsis and trigger apparent deformities, which are reminiscent of a pulsating amoeba or the movement of a jelly. The motion detectors of our brain are only able to register the speed components vertical to the red line. There is freedom of interpretation of the movement. In reality, two rigid systems, i.e. the ellipsis and the system consisting of the four black spots, should be observed simultaneously. But because the second system is not separated from the first, our brain constructs its own system with flattening movements.
It is acknowledged that our biological clocks tick at various speeds according to any given situation. Our brains produce a timing schedule as a way of ordering our perceptions. Our biological clock is not the same as physical time. If, for instance, a live tiger crosses our field of vision, our biological clock ticks relatively slowly. Our perception intensifies since we are dealing with a complex moving figure. However, if a rigid mock-up of a tiger is pushed across our field of vision at the same speed, our biological clock ticks somewhat faster because this less complex figure is comprehended more easily. The four black spots on the red ellipsis of pendulum 2 move relative to the ellipsis, thus increasing our attention and creating a slow motion sequence.