it doesn't have to be a belt, it could be a fully hydraulic unit. engine drives a variable displacement hydraulic pump (wobble plate type) that produces pressure to drive a hydraulic motor coupled to the drive shaft.
When the speed is low the rake angle of the hydraulic pump is small, making the pistons take a short stroke thus making the system pressure, but at a low volume, and as speed increases the rake angle of the wobble plate increases to accomodate the additional volume needed at higher speed.
This type of system has been being used ever since the 60s in turbine engines. They are called CSD or constant speed drives, and are used to turn the AC generators to supply AC power. In order to supply a constant frequency the speed of the generator is held constant by the CSD while engine RPM varies from idle to max RPM.
