Why no negative pitch?
My understand is that helicopters can not select negative pitch, in the late 90's when I used to fly very large RC models we had this. The mechanics of our large two stroke models matched the mechanics of full size machines.
When on the ground with a high rotor rpm that was decaying dropping the throttle/collective to lower limit introduced a degree of negative pitch as a safety measure as a decent gust of wind could get you airborne again whilst you were distracted. Negative pitch ensured you stuck to the ground.
But also in auto rotations, again my understanding of helicopters if the engine quits you quickly drop the pitch to maintain rotor rpm relying on the inertia of the rotor system to provide the flare as you land, if you could introduce a degree of negative pitch you could build rotor rpm as you descended.