Governors are devices fitted to machinery to ensure smooth operation. They consist of a spindle and two cantilevered arms each with a heavy weight attached to the end. The way they operate is that a governor is attached into the power train and then spins. The faster the spin the more the weights are moved away from the spindle by centrifugal force. This movement can then be translated to machinery controlling mechanisms smoothing out fluctuations in power.
The windmill's governor makes use of this power smoothing and applies the results to the Millstones. It works like this, as the Main shaft rotates faster the Governor alters the distance between the two Millstones thus ensuring a more constant milling speed, and better resultant flour.