Page "Rubik's Revenge" Paragraph 9

The original mechanism designed by Sebestény uses a grooved ball to hold the centre pieces in place.

The edge pieces are held in place by the centres and the corners are held in place by the edges, much like the original cube.

There are three mutually perpendicular grooves for the centre pieces to slide through.

Each groove is only wide enough to allow one row of centre pieces to slide through it.

The ball is shaped to prevent the centre pieces of the other row from sliding, ensuring that the ball remains aligned with the outside of the cube.

Turning one of the centre layers moves either just that layer or the ball as well.