A moderately quick search reveals a few pprune threads and an accident report. From these I understood that:
The hydraulic jack stall occurs when the aerodynamic force on the rotor blades exceeds the force the hydraulic actuator can apply.
The actuators are controlling pitch of rotor blades and are applying force to keep the angle of attack high. When the angle of attack gets too high the blades stall which is associated with change in centre of lift creating a moment trying to rotate the blade to lower angle of attack, strong enough to overpower the actuator.
The right pitch moment is because the retreating blades have higher angle of attack and approach stall earlier.
The forward pitch moment is when the stall happens in higher G manoeuvre where the helicopter is pitching up and thus the blades in front are running at higher angle of attack and approach stall earlier. If the stall happens due to overspeed, the pitch moment will be back instead. In either case it will be opposite to the selected cyclic.
The down collective moment is because the angle of attack decreases on the forward (or backward) and right blades, but does not increase on the other ones, so the total lift is reduced.
The behaviour can be considered a good thing as it prevents aerodynamic stall from fully developing. The effect is also similar to what would happen if the actuators were more powerful and aerodynamic stall did fully develop. It can however be more surprising for the pilot since it may happen before vibrations that normally indicate incipient aerodynamic stall. The condition is self-correcting in that the reduction in pitch causes reduction in G loading and thus reduction of the aerodynamic forces, but the drop in performance can obviously be a problem if it happens too low.
