You need to look at the cross. In this demo, there are three moving bars with different luminance (depending on your screen maybe you cannot perceive the dimmest one on the top but just perceiving two is ok to get the illusion).
The illusion is that although physically the bars are aligned, the brightest bar seems to lead the dim ones. Notice that the spatial misalignment that characterises this illusion is small.
The explanation is that the brightest bar seems ahead in the direction of motion because the neural delays processing it are shorter that for the dim bars. Neurons respond faster to high luminance objects than to dim objects.
We show that the illusion does not happen when people try to synchronise an action with objects with different luminance.
Visuomotor timing compensates for changes in perceptual latency. White, A. L., Linares, D., Holcombe, A. O. Current Biology,18(20), pR951-pR953. (Supplemental data)