Well, as web designers will know (see our pick of the most beautiful parallax scrolling websites), it's when the foreground and the background move at different speeds, creating a 3D-like sense of depth. In this real-life case, it's the position of the viewer and how they are moving around the plane that makes it look like it's standing still.
Another real-life illusion captured by a bemused onlooker was this flying boat, which we also highly rated, even though it didn't have links to web design.
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