Yet another process is usually to use a variable displacement pump.  In the event the pump is configured for reduced displacement, it creates a minimal volume of oil movement, creating the hydraulic motor(s) to turn more slowly but surely. Merge harvesters utilized variable belt drives as early as the fifties. https://gearbox-cvt97520.actoblog.com/33350806/a-secret-weapon-for-gearbox-cvt