Being in the desert environment in Southern Arizona, I stop and inspect every windmill I come across when tooling around in the boonies. I want one for my front yard, but can't afford one yet, and most of them are full of bullet holes. Aermotor seems to be predominant out here.
The centerline of the sail or wheel is offset from the centerline of the mast. This offset gives a torque reaction when the wind gets too strong and pushes the sail even more off center causing it to skew. The tail simply follows along, with the tail spring always trying to return it to center. When the wind dies down, the tail spring will bring the wheel back to facing into the wind. The big knowledge or calculation is how strong the make the tail spring.
Hope this helps.