Uh, Oh, I might have to agree with Lu on this. heedm, note the date and time!
Jack stall is a sign of a servo system with too little force capability, and is not acceptable in modern helicopters, as a rule. It is caused when the stall of the retreating blade causes the blade to change its pitching moment sharply downward, which makes the blade try to dive. The strong downward force is fought by the pitch change links, swashplate and servos. At Sikorsky, we set the design requirement of the servos so that with one stage failed, the other has the full capability to fly the flight envelope. That means that we have twice as much servo force capability as needed in normal unfailed circumstances.
I personally set the S-76 flight test points to prove this, with max weight, one servo turned off, we pulled over 2 g's at Vne with no change in stick feel.
A helicopter that shows jack stall is on the edge of taking over for the pilot, and turning him into a passenger for what could be a very short ride. Treat all jack stall cases as bad ju-ju, because the difference between a stick that throbs a few cycles, and one that thrashes around and allows the aircraft to go ape is a very small difference in maneuver load factor.