The ball rests within a metal ring fitted with ball bearings. The metal ring fits within a plastic base, and is free to turn within the base. This allows the ball to be turned in the cup without having to lift the ball.
I got the chance to try one out at a local shop the other day, and it makes working out thumbholes *much* easier.
Very cool!
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Horrid in Doubles, torrid in Singles...
...that's The Curse of Dusty.