The prop / rpm governor still functions as normal. It flattens the pitch to maintain the same RPM, unless you are very slow and the min angle of the prop is reach, the same rpm will be maintained.
HiTech
I figured the governor would still be at work because I'm not sure how it's actuated, electrical or hydraulic, but that shouldn't stop the engine from slowing down. There's too much internal friction and compression in the engine.
In any case, it means nothing because I'm not even sure how much of a delay there is or how significant it is compared to just throttling one engine. My guess was that throttle provided quicker recovery of thrust.