I've found that this optical illusion works best if you have the screen far away from you, or if you allow your vision to unfocus.
I suggest doing the former, as doing the latter gave me a headache.
If you do, after you hit the Start button, it should appear as though the two horizontal bars in the canvas are taking "steps" one at a time, even though you can verify that they are not by clicking the Toggle Contrast button.
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