This is a very poorly ( in other words not at all :P ) communicated point, both visually and design-wise (might not even be another puzzle that can be blocked that you put puzzles on now that I think about it?), but the blockers do block the movable panels from "docking" to the underlying panel, and that's what happens here. It still snaps whether or not it docks, I had ideas about doing something using that but never got around to it so would probably be better to have left it not snapping at all...