What is it? You see (or rather perceive) a tilted line (tilted upwards), even though the dot is moving horizontally in raster scan fashion. If you are not able to see the illusion clearly, set the frame duration equal to reciprocal of your monitor’s refresh rate. Example: if your monitor’s refresh rate is 60Hz, set frame duration = 1/60s = 16ms. You can also try integer multiples of 1/(refresh rate).
Take time to absorb the illusion, and then proceed to explanation page.