Thank you Shy and Rotorbee, that is what I was asking - not that a guy screwed-up, we can see that - but the physics of what happened as applies to helicopters.
But I am still not quite there in my understanding. Suppose there been a helipad near the top of that ridge, are we saying that there would not have been enough power available to come to a hover and land on it? i.e. was the helicopter was above it's operational ceiling in terms of performance? meaning it could only stay aloft with forward speed, but did not have enough power at that altitude to hover?
Because he clears the ridge, and only crashes on the other side when the ground falls away. Was it because his Nr was reducing and bleeding energy as he approached the top of the ridge, and he barely got over it, by which time his Nr was too low to be recovered by engine torque?
In which case, once clear of the ridge could he have relaxed the collective and descended to follow the descending terrain to gain forward speed and lift?
(I am not advocating this as a sensible way to fly routinely !).