You will have to forgive me a lot here for my apparent ignorance, but I was under the impression that a governer does infact react to the speed of the engine as (again only an assumption) it uses a magnet atached to the fan and a sensor to count revs. If this is the case then it is looking at engine revs not blade speed then changing the throttle amount to compensate and keep the speed the same under all loads.

I have no experience with a governer at all other than reading mags and sites so I cannot in any way give a real oppinion as to how good or bad they are or even how they actually work.