Generally the horse hits a fixed jump with the front leg(s) between chest and knee when going at speed. The momentum of the horse keeps it moving, so it pivots (rotates) over the jump, firing the rider into the ground and then potentially landing on top of the rider.
I fence judge for BE - I've seen a rotational fall. The horse went vertical but head down, tail up, then landed on it's back. Fortunately the rotation was at a slight angle so rider was to one side of where the horse landed.
Potential causes of a rotational include ,Hind feet slipping on takeoff ,or deceptively deep takeoff surface.Both cause the horse to fail to gain height and as Magnetic sparrow has so excellently explained , the horse hits the fence above the knee and rotates . Another killer is the unfixed xc fence especialy chairs. If the horse hits the fence ,the fence moves with the horse and he cant scrape over the top and get clear. There was a very very brave and heart rending post on the internet ,by the parents of a girl named Elenor (I will never forget her). Her parents posted the video of how she died due to a rotational at an unfixed chair fence. They wanted the world to see and learn ,god bless them.And now I am snuffling into a hankie cos men aint supposed to cry.
Couldnt get his back legs under himself ,so couldnt get the height . A brave horse is more likely to give it a go .The saner horses would simply jam on the brakes.
I've seen only one rotational in real life (I hope never ever to see another). The horse and rider came into an open V shaped fence, lovely stride, lots of energy but for some reason the horse just didn't stretch out enough over the fence and put his front legs down in the middle of the fence so flipped up and over it, thankfully the rider was thrown clear, the horse was badly winded but fine.
I had one weirdly enough show jumping in June whilst doing a BE 100. Luckily I only ended up with broken ribs and a bit battered bruised.
To this day I don't know what happened as had a perfect stride, not too fast or lack of impulsion. The only thing I can think of is that she slipped as she pushed off and somersaulted.
It was terrifying watching half a ton of horse roll over the top of you and not being able to do anything about it.
I've had a rotational fall at an unfixed portable fence. Scariest thing; was so relieved when my horse got up - using my leg as an anchor - thank God for my long leather boots they saved my leg from serious damage.
I was very lucky & thank my horse every day as she was so calm & just rolled over got up & stayed with me until I got my breath back. I got back on & jumped a little log & called it a day. Very lucky to have a horse like her
I had 1, only time I have ever fallen off my mare. It was horrible.
I came into a 100 fence it was a gate style but had about a foot of width (weird fence!) perfect stride then mare my faltered then decided she would be brave & take it on. Hit 1 of her front legs off it & tipped me straight onto the fence then over, she kept going & was very nearly over but got stuck on the fence & kind of paddled back over again. Luckily she was fine. I thought I had broken my arm, I couldn't breathe or move for about 5 mins but after a while of being told to lay on the floor I got up & was ok.
I had a "rotational light" over a tiny showjump earlier in the year. Pony just didn't quite register it. He did go over and land on his back, but he ate dust first (as opposed to Buzz etc who flipped over the jump itself and landed on their backs).
Often a horse hits a fence and there is a good chance that the rider will be thrown clear, however in a rotational fall it is all too easy for the horse flip over and to land with the rider underneath.