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If you follow the video’s instructions, when you look away you will continue to see wavy lines in your wall or on the floor. This happens due to an optical illusion that is the result of repeated psychological stimulation. When the video ends and you look away, your brain still expects to see the waves, and therefore it creates them for you. Saying the letters out loud doesn’t really play a role, it just ensures that you are focusing on the center of the screen, where you can best receive the stimulus.
WARNING: Please use your discretion when viewing. If you suffer from photosensitive epilepsy, please do not view this video.