My only experience of jack stall was in a single engine helo at MAUW, which occured over 25 years ago. We were asked to carry out a single pass over a military parade ground having just disembarked from 'mother' a few miles away. Approached the target downwind in a slight descent for maximum speed (must have been approaching 95 kts, at least), as we on topped the target, the aircraft was pulled up and right and instantly the most violent vibration was experienced.
The aircraft then continued on the original approach hdg in a slight descent, as opposed to the control input of up and right, (probably because the controls were relaxed!) the speed washed off and the vibration stopped. Ht loss was maybe 50-100 feet, which was OK because our escape route was over ground that dropped away. We made our excuses and left.
I have experienced many flight regimes in helicopters with nothing scary happening, but the factors on this occasion for onset of 'Jack stall' were (I believe), very heavy aircraft, in a descent for max speed with collective applied and forward cyclic, and the application of more collective and cyclic to pull up and right which resulted in a lot of 'G'.
Certainly caught us out, and it never happened again!