The important thing about zero g is that it creates, for teetering rotor heads, zero cyclic control. That is because the only means for a teetering rotor to control the helo is to redirect the rotor thrust. No thrust, no control, no hope. A contributing factor is that when the helo has no control, as the helo is departing level flight the pilot moves the stick all over the place and that makes the head bump the mast. After mast bumping, the weakend mast lets go and then the head removes itself, a very embarassing event. It is often said that low G makes mast bumping. Well that is as correct as saying that teeth give you cavities - correct but not nearly the whole story.
The real problem is that every helo guidebook for neewbies and instructors is fundamentally wrong in how they explain cyclic control, so most helo pilots get confused as to how a teetering helo has this Low G problem when the others (articulated or bearingless/rigid) don't. Helo "technical" guideooks, like the FAA Handbook, all show that the cyclic simply tilts the thrust, and this makes the helo tilt and this gives control.
Articulated and bearingless/rigid rotors derive most of the cyclic control from the fact that they make the helo rotate about the mast just due to the strong moment they produce due to flapping. This strong moment control is available even though there may be no rotor thrust - in other words, most helicopters have excellent rotor control at zero g, except teetering rotors. The measure of the amount of moment control that a rotor head has is made by ratioing the spanwise distance of the flapping hinge to the total radius of the rotor - spoken of in percent. A typical articulated rotor has about 4 to 5% "hinge offset". How does a bearingless rotor get this moment control? It bends as if it had a bearing, and has a "equivilent offset" that is usually even better than an articulated rotor, typically about 7 to 11% hinge offset for a bearingless. This hinge offset is why articulated rotors feel snappier than teetering, and bearingless rotors feel even snappier.
Now is autorotation a zero g event? No, the rotor produces a ton of lift in autorotation, equal to the weight of the helo. In entry, if the pilot slams down the collective, there can be a bit less than 1 G for a fraction of a second, but even that is not zero g. In short, unless the pilot slams down the collective, mast bumping is very unlikely in an auto.
