How does the Shtox glass spin?

Of course, the Shtox glass is not a perpetual motion machine, and it does not  spin due to some unknown supernatural force. To get it started you have to spin with your hand. But still... all the same - why does it rotate, unlike all other tumblers? The secret lies in the unique, specially designed shape of the bottom of the Shtox glass.

An expert grinder handcrafts the bottom of each glass to the right specifications. The high precision grinding allows the Shtox glass to spin, and at the same time, not to sway from side to side.