"It means the governor will not be effective, the engine can be reved faster, maybe even beyond the safe limits for that engine.
I suspect that the governor settings of WW2 US trucks were set artificially low, therefore, a truck with governor disabled could still be operated at somewhat higher revs by a careful driver without problems.
But, I have heard that on some engines, even a few hundred RPM over recommended has dramatically reduced engine life.
My 2 cents.
Sam".
A WWII GMC trucks forum friend answered me today.