But how do they work?
bujhm tried all ways to find it out - meditation, internet search et cetera - and finally come with a clear-cut solution (by a metal saw).
He discovered that:
Each ball welded from two hemispheres and then polished
A metal spiral inside is welded by its end to the bottom of one hemisphere
A little ball with flattened top roams free over the spiral
The sound is produced by the spiral and then resonates in the big ball body.
The tone depends on which part of the spiral is hit by the little ball.