I think if horses gave out awards for MIH (Most Important Human) Gerd would certainly be in the running.
As for sliding stops with reining horses, having done a great deal of it back when head down, looped reins, were uncommonly seen in stock horse contests, I have some comments.
The stiffness of the back depends on the rider, the horse's training, and the horse's capacity athletically. The shoeing and ground preparation matters too a great deal when it comes to safe and well executed slides.
I've worked horses that could not keep from lockin up in front. And others that could "walk" through a spectacularly long slide. Obviously the latter had the most flexible most well flexed back, as in "rounded-up." With their head down.
But the worst abuses I saw had to do with repeating hard sliding stops far too frequently. Those horse's always burnt out (as in lost their ability to slide) and often suffered injuries.
My own training routine in the past, and what I see as best practice currently, is to train for the big long slide, but perform it rarely. That's how and when the truly beautiful slides can take place.
I focused on voice command only and dropping the reins as the goal. In other words, cues that release rather than restrain.
Let the horse position his own head. If I'm balanced laterally, get my back pockets down into the saddle, keep the heels down, and sit straight up, the horse can do the rest. And will.
A "stiff," back isn't a fault. It's a momentary tensioning of muscle and tendon. As with any athletic movement. It's using the muscles, bone, tendons, ligaments.
Because I came from stock horse, working on a ranch, through racing I got very into how the horse uses himself. And what conditioning he must have to do so with grace and power. And what kind of riding and training (as in "cuing") brought out the best with the least chance of injury.
And questions of "collection," weren't given much thought. What was the focus was "range of motion." What I think of now as both extension and collection as the horse goes through certain activities.
And what is even more fascinating and not well explored I think is that the horse has two ends.
That is, as one sees in a well executed slide, a well under haunches, but and extended neck and front legs reaching forward. Being extended and collected at the same time.
In fact, the slide, to be acceptable, must be executed from a gallop, and extended gait, into the collection of tucking under the haunches and bringing the hind legs well under and holding them in place, nearly still and braced -- which is about as 'collected' as one end of a horse can get, while keeping the forehand open, moving, and active, which is about as extended as the other end of the horse can get.
Which is what I refer to when I point out "range of motion," should be the goal in so much performance work.
I ache physically watching modern Dressage because of so little actual extended work seen. Even the "extended" trot is not. Unless both ends of the horse are "extended," it's simply a contortion of collection.
Unless Gerd is incorrect (and I do not believe so) the horse cannot flex and lift the belly and raise the back (by bending around it -- you are right it doesn't raise, the rest of the horse simply lowers) with the angle of the neck high in rollkur.
If I had my way, being Emperor of the Universe, the horse would be conditioned through range of motion and all performance work would be around athleticism that restricted events to activities that required
range of motion. And judging would be based on that capacity to use their entire body range.
It is coming to that.
And if horses were bred for it we would see a return to the horse as it existed in the wild. Where the ones with athletic full range of motion were the dominate type.
Donald