The blue and yellow arrows in this video seem to move up and down in a rhythmic dance—yet their motion is strictly horizontal and uniform. This illusion is a variation of the “Stepping Feet Motion” effect, first demonstrated by Stuart Anstis in 2003.
In the original phenomenon, two objects glide side by side at a constant speed across a striped background, but appear to alternately speed up and slow down, creating a stepping or oscillatory effect. The visual system misreads contrast differences as changes in motion, producing a false sense of vertical or staggered movement.