This video shows the following process in reverse order. The person who made the video spent hours placing tiny white squares at specific points on the checkered pattern. The spots cause viewers to see the the pattern as being warped, because the brain and eyes pick them up as visual cues, and we can't help but connect the dots, which lead inward toward the center. So, our perception is skewed. At the end, he blew the dots off the pattern to show that the pattern was actually parallel. Watching it slowly in reverse shows the effect of the illusion as it skews the viewer's perception.