Correct classical riding only occurs when the rider has a good seat and a correct and well-balanced body position, moves with the horse's motion, and applies and times the aids correctly.
The origins of classical dressage and collection lie in the natural ability of the horse and its movements in the wild. In fact, most modern definitions of dressage state that the goal is to have the horse perform under saddle with the degree of athleticism and grace that it naturally shows when free.
Horses naturally use collection when playing, fighting, competing and courting with each other. When trying to impress other horses, they make themselves look bigger, just as other animals do. They achieve this by lifting the forehand, raising the neck and making it bigger by flexing the poll, while at the same time transforming their gaits to emphasize more upwards movement. When fighting, the horse will collect because in collection he can produce lightning speed reactions for kicking, rearing, spinning, striking with the front feet, bucking and jumping.
This natural ability to collect is visible in every horse of any breed, and probably inspired early trainers to reproduce that kind of behavior in more controlled circumstances. This origin also points out why, according to most Classical dressage trainers, every healthy horse, regardless of its breed, can perform classical dressage movements, including the Haute Ecole jumps, or Airs above the ground, even though it may perform them a little differently from the ideal performance due to the build of its body.
The ultimate goal of dressage training is to develop a horse to its ability as an athlete: maximum performance with a minimum of effort. The training scale (as set for in the German riding instruction) is to physically develop the horse in a consistent manner with longevity in mind. Dressage is fitness training and needs to be treated as such, with thought, compassion and patience.