Armchair Flyer, what i have been taught is that for the rotor to keep producing lift it needs to keep turning. Seems pretty sensible to me. And for it to keep turning it needs g loading. 1g will do nicely. 4g turns it faster. There is an upper and lower limit of rotor speed..... negative g is thus bad news.
Indeed, RRPM is life as some rotary pilots put it. And apart from the risk of excessive flapping, stick forward tends to reduce RRPM in helicopters, too (and vice versa), so another instance where a reaction often helpful in fixed-wing aircraft might be unfavorable in a rotary aircraft, even without brisk and exaggerated control input (such as in a low-g pushover).