It’s essentially a lens that spreads out the light horizontally, so any point sees a blur of everything on a large horizontal line. The person is only a small part of that, so is mostly invisible.
Works best with a background that varies vertically but not horizontally. Also only works great from certain positions. See this video for moving around similar lens. Also check the laser-pointer section in the middle to see the light path.
[https://youtu.be/wsR14D5FzsM](https://youtu.be/wsR14D5FzsM)
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