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, so we are stimulating low-level neural motion detectors. Below: A spoke appears to move only when a sector of the same luminance effectively jumps across it.
As a demonstration, the disk jumps back and forth through one sector width (darkest sector made green to show better). Look carefully within the red rings, where the spokes are the SAME grey as the sectors that flank them, and you will see the sector borders move OPPOSITE to the overall sector movement. The locus of these tiny counterclockwise movements runs clockwise around the rotating disk.