The sectored grey disk steps around clockwise. The thin grey spokes never change their brightness or position, yet they appear to drift around counterclockwise. Gaze at the centre for 20s, then stop the movie, and you will see a clockwise motion aftereffect in the spokes. A spoke appears to move only when a sector of the same luminance effectively jumps across it.
The disk jumps back and forth through one sector width (red line). Look carefully and you will see the sector borders move OPPOSITE to the overall sector movement.