Gaze at the two black fixation marks in turn.
Bottom left (control condition): The surround is darker than the drifting checkered lines, which are seen normally.
Top right: The surround luminance lies between the luminances of the spots (checkers) that constitute the lines. Now the spots appear to trickle non-rigidly along the lines, as though the spots are moving faster than the lines. Explanation: When the spot luminances straddle the surround luminance, the contrast along the lines (between adjacent spots) is higher than across the lines (between the surround and the spots). The motion with the higher contrast wins, and motion along the lines predominates over the sideways motion of the lines.