Centralize has it a bit off, let me discuss:
Jack stall is caused by rotor stall. The airfoil on the blade shifts its moment to pitch the nose of the blade down at stall, just as the wing of an airplane retrims in stall, and the nose drops. This stall sends large forces into the helicopter's rotor system, trying to force the servos backwards. If the servos are of marginal design, they can be over powered by the stalled blade, and this backdrive is felt by the pilot, in fact, the helicopter gets a mind of its own and is actually out of control. This jack stall is the natural product of the combination of a stalled rotor and a marginal servo system.
Low g is a problem for teetering/underslung/semi-rigid rotors, but is duck soup for other designs. The explanation is that the rotor does not control the helicopter purely by tilting the lift, as is often said in helicopter primers. The moment produced by the rotor is more powerful than the lift tilt, but this factor is almost never explained.
In an articulated system with hinge offset of 4%, about minus 1/2 g is where the controls have little effect, and for rigid high offset rotors, it can be minus 2g's or so before they have no effect.
Nick