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Here's how this illusion works:
This view of the staircase reveals that the connection between A and B is only an illusion. With a 2D representation of the staircase, it is possible to choose exactly the correct point of view that would hide the gap between A and B. Without the beam in the picture above, there would be no way of knowing whether B was one step up from A, or the continuation of the staircase that wraps around the front of the picture.