I have practical experience of this. During the latter part of the 80s I was on task over Belfast,with some of the you know whos, doing you know what.Arriving back in the the high hover (after a racing refuel at Palace bks)and over the less salubrious parts of west Belfast, it all went a bit "Pete Tong". High auw 1800kgs (or thereabouts) in a Gazelle, low airspeed, high power setting, light winds aloft (10kts or so). Suddenly it didn't feel right, and concurrent with that thought, the VSI went almost instantaneously beyond the vertical - downwards !.
Oh Bum. Did two things pretty much simultaneously, cyclic forward, and dumped the collective. The aircraft pitched over slooooowly, and airspeed began to build - slooowly. However West Belfast grew very quickly in the windscreen, interesting details like nails in roof slates, worn tyres on cars, man scratching bum etc.
Chappie in LHS asked me what I was doing (not an unreasonable question) as he was fully expecting to be eyeballing "Bill Baddie", and not the rapidly approaching road features, and general lack of hospitality that the local residents had against her majestys government - at that time.
Bottom line. Fell about a thousand feet, recovered, staggered back up to task, genuine white knuckles.
Post task, had a cup of tea - and got a bit introspective.
Root cause - Rushed approach to task.
What saved me. Demo of said condition in basic rotary (1983 at MW), and stall practice in "Chippies"
Not big, not clever.
Stay safe