Demonstration of a variable speed pulley.
The sheave on the right is attached to the control lever. Spreading the halves apart or pressing them together effectively controls the diameter of that pulley. The pulley on the left is spring loaded. As the belt is pulled tight by closing the control pulley the drive pulley spreads and effectively becomes smaller. Conversely as the control pulley is opened, the spring tension closes the drive pulley, effectively making it larger.