For any who never had the pleasure of meeting this self abuse toy. It was two balls on a string. They balls were made of various material I recall mostly glass,marble and wood, I had a steel set. The idea was bounce the balls together build speed and double clack them. Once above once below, not rocket science but hard to do for a long duration!
OK I have to ask who has blackened their wrist in the attempt to get a few hundred hits in a row. Or perhaps you were like me and missed and severely beat the Glass or in my case steel ball against some part of the arm hand or wrist!.