Ok, so I don't think I have started a thread yet, so here's my first attempt.
People talk a lot about getting a "bend" in their horse, and especially in the rib cage or back. But from what I have studied of horse anatomy, it is physically nearly impossible for a horse to actually BEND in its shoulder, thorax or pelvic area, at least lateral bend. There have been studies with photos taken from above of riders "bending" there horse, and just riding straight and the horse's body doesn not percievably change. What does BEND, is the neck, all the way from the poll to the front of the whithers.
So, Why do we keep perpetuating the belief that we can bend an animal that CANNOT bend in his body this way, physically cannot bend due to the rigidity of its spinal column. It can, however, bend longitudinally (i.e. It can tuck it's pelvis under, a very small amount . Once can easily see this when the horse manures. It can rotate the pelvis and somewhat the spine, but not a whole lot.
I wonder if some people are fruitlessly trying to bend a part of the horse that cannot bend.
What does happen, and it feels like bend to the rider, is that the barrel of the horse "rolls" right to left with each step under, and if a horse is asked to engage his hind leg further under the body this will cause a more distinct barrel roll of the large abdomin.
So, I would love to hear what others think of this. Kind of a vague thread starter, I guess. I am no pro at this.