I've recently had a little epiphany about collection.
The great desideratum of collection for both humans and horses, is release at the poll, aka the atlanto-occipital joint.
As an Alexander student, and teacher, I have spent many hours, days, and years attempting to release my poll or to induce others to do so. I am all too aware that these efforts are often forced, and actually involve a stiffening of the neck and, rather than a dropping of the head from the atlanto-occipital joint, more of a pushing of the head/neck forward from the region of the seventh cervical vertebra (C-7).
A true release of the neck involves a dropping forward of the front of the head, while the back of the neck below the poll actually releases backwards.
The same is true of the attempt to collect horses. The horse's head should "nod" forward from the poll, while the rest of its spine should release toward the tail, allowing the hind legs to come under. When you look at photos of dressage horses, it sometimes appears that their necks are drawn forward towards their heads, in a misguided maneuver similar to the tendency of humans to push their heads forward from C-7. I have been guilty of inflicting this on horses myself.
The term flexion is itself a misnomer. To flex implies muscular effort. Whereas collection begins above all in a release at the poll.
I have been struck, looking at KFH's horses, how upright their necks seem compared to what I'm used to. I believe this is because they are truly releasing at the poll, and their cervical spine is able to release back toward their tails, which allows their quarters to drop. Another horse which has struck me in this same way is the amazing palomino bullfighting horse Merlin. (Not that I'm condoning bullfighting.) He displays intense collection, and usually on a loose rein.
Many photos of competitive dressage horses suggest a horse pulled in two different directions - from the withers forward going one way, the quarters going another, and the middle lost in between. They can't really "sit down," because their necks are being pulled away from their tails.
The other difference which strikes me is that while competitive dressage horses' profiles are typically vertical, or behind the vertical (nose behind eyes), Klaus's horses and Merlin have profiles which are ahead of vertical (nose ahead of eyes).
I think it may be possible to use reins to ask for or suggest a release at the poll, but it is a contradiction to compel a release.
In Dancing With Horses, pp. 56-57, Klaus has some interesting observations about how we attempt to force the horse into a frame resembling collection, and in so doing lose the essence of collection. He believes that true collection originates in the hindquarters. I'm not so sure, because I'm pretty certain that in humans it originates in a release at the poll. Either way, it can never be forced.
One of these days, I'm going to find myself on the back of a horse again, and I'm going to try to find a way to discuss these issues with the horse.