I have a question I've recently been wondering about: why, in a lengthening, is the neck and body not allowed to lengthen? (According to most of the instruction I come across these days.) It seems it would be a much better exercise, gymnastically-speaking, than just the extending of the shoulder/legs.
In my own schooling, the long-and-low and really helps to develop into working/semi-collected work, and when I start lengthening, it's an exercise for my horse to reach into the bit, thereby getting longer. The stretching, then coming back, then going low all seems to get her more supple and looser.
Of course, she does not have the gorgeous movement of the horses in the video. Do you think that makes a difference in how to go about schooling?