Set Ab bridges A and B. The page is no longer one texture but separate figures that belong together, and one is missing. Sometimes the missing figure is simply the same as the others. More often the figures are parts of a related whole: arrows pointing in, bars in a pinwheel, corner triangles, orbiting pairs, and the missing piece is the part that completes the whole.
What you see: figures that belong together, one gone. What you do: say what the whole is, then what the gap must contribute to it.