devil 49,
The reason for the jack stall is that the blades are stalling, and when they do, they progressively add more and more force to the swashplate. This is due to the pitching moment of the airfoil shifting with the stall. When the swashplate gets heavily loaded, it pushes the servos backwards despite the hydraulic pressure in them. this is called "transparency" or "stall" of the servo.
here is a web site that talks about the dynamic stall of rotor blades. Scroll down to where they talk about the pitching moment (the Cm plot) to see how the moment of the blade takes a big turn at stall. This is the reason why the nose drops on an airplane when the wing stalls - the wings pitching moment shifts sharply downward.
http://aerodyn.org/Dstall/dstall.html