More than 25 years ago I was involved in a covert military role which required a "free air" OGE hover by night. OGE in that context meant up to altitudes where oxygen would ideally be carried. We didn't have oxygen so that was one of the things we had to be aware of.
At those altitudes there were no accurate visual references so we had to learn how to achieve a hover on the normal flight instruments. To hold a fixed position over the ground the aircraft had Doppler tied into the artificial horizon, which had a lateral/fore and aft needle presentation similar to ILS on other aircraft (the aircraft we flew didn't have ILS fitted).
A critical instrument was the VSI. The ASI stopped working below 40 kts and we had no performance figures in the manuals; so when the people we were working with wanted maximum altitude, we had to attempt it on a try it and see basis. We were working at the limits of the aircraft's performance (and ours). Once in the hover I used to set maximum continuous power and see if the aircraft was climbing or descending. If it climbed ( it seldom did) I let it do so and find its own maximum altitude. We might be hovering there for a couple of hours.
The part relevant to this discussion is that sometimes we had literally aimed too high and the aircraft simply didn't have enough power to achieve a hover. If we persisted, with airspeed "off the clock", the aircraft would begin to descend, despite full power being applied. I would call this "settling with power". It was recoverable by lowering the nose, flying away, then descending 500 feet or so, then trying again. At no stage was control lost.
However, on one memorable occasion, after a very long night and approaching first light, we were all very tired. The handling pilot momentarily lost his concentration and after a prolonged hover, things suddenly went very badly wrong. I believe the aircraft gained a slight negative airspeed and this was coupled with an increasing descent, despite full power. The aircraft suddenly began randomly pitching nose up and down (nose above and below the horizon) and rolling left/right and I noticed the VSI needle had hit the bottom stop (2500 fpm). I called "Airspeed, Airspeed!" The HP didn't respond at first, so I repeated the call and "assisted" him to apply full forward cyclic. The aircraft was slow to respond but very suddenly did so and we flew away, the ground not far below us. We had lost a lot of altitude, thousands of feet. Now THAT was certainly vortex ring state. The difference being that the aircraft didn't respond to normal control inputs until airspeed was regained.