A rotating concave grain threshing system has a rotor positioned within a concave. The rotor and concave are each rotatable in different preselected speeds and directions through respective transmissions connected to a power source. Measurements are taken corresponding to a physical property of the crop being harvested and the torque induced on the concave. A signal is generated and is responsive to changes in the measurements and delivered to a controller. The controller will then create a controlling signal corresponding to a difference between the desired physical property and the measured physical property and/or the desired torque and the measured torque being greater than a preselected amount. An actuator receives the controlling signal and changes the speed of the concave in response to the magnitude of the received controlling signal.