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You can see the inner thread in the pictures. While turning a side the central element goes up and down by itself. There are several types of the central elements: 1) 90 degrees left/right turn, 2) turnover, 3) 0 degrees or complete revolution.
Because of this, turning of sides has some limits, which influence the complexity both of Crazy and Bermuda set.
One can create a pretty many different puzzles, combining different centers and contrive another set of planets.