Keep it simple – on a teetering head helicopter the fuselage dangles underneath the rotor head and where the rotor head goes, the fuselage should follow. You move the cyclic to the right, the rotor disc tilts to the right and pulls the weight of the fuselage along with it until the rotor and fuselage are in their normal positions relative to each other except in a right turn.
Now remove the weight of the fuselage – ie zero g, it still has mass but no force acting on it – the rotor will still respond to cyclic inputs but cannot change the attitude of the fuselage – think of the rotor mast as a piece of string that needs to be pulled tight.
On a helicopter with any hinge offset ie not on the axis of rotation, the rotor hub has some leverage so it can make the fuselage follow it even in zero g.
So in a push over / low g situation in a teetering head helicopter, the only rotor capable of affecting the fuselage attitude is the tail rotor because it is fixed to it. Unfortunately this is pushing to the right producing anti torque thrust which will tend to yaw the fuselage and roll it (secondary effect of yaw) aided by the TR thrust creating a moment about the fuselage C of G.
The result is mast bumping as the pilot tries in vain to control the fuselage attitude with the cyclic or worse still, the MR and tail boom coming into conflict and the MR usually wins.