Jack stall is associated with the hydraulic servos/jacks that operate the main rotor blades. In certain helicopter(AS350/355, Gazelle are ones that I know of), during hard manouvering, the aerodynamic loads on the blades can become high enough to prevent the hydraulics from being able to move the blades further.
In the Gazelle, there is an uncommanded pitch up and roll to the right. Simply reducing the severity of the manouver will reduce the load and free the controls.