What is it? You see (or rather perceive) a tilted line (tilted upwards), even though the dot is moving horizontally in raster scan fashion.
Take time to absorb the illusion (don't change the slider), and then proceed to explanation page.
Requirements: You should have Silverlight 2.0 or higher installed on your machine, and your machine should be able to sustain the display at 60fps. The illusion may not work at low frame rate (30 fps or so).