What is the difference between collection and compression in dressage?

The distinction between genuine collection and mere compression is one of the most important technical concepts in dressage and one that separates training that develops the horse correctly from training that produces the appearance of collection while actually working against the horse's development and welfare. Genuine collection is the horse's ability to carry more of its own weight on active, engaged hindquarters — the hind legs stepping more deeply under the horse's center of gravity and bending more at the hock, stifle, and hip to carry the load rather than push it, which simultaneously lightens the forehand and allows greater elevation and expression in the movement. This genuine collection requires significant muscular development of the hindquarters, loin, and back, and it develops gradually over months and years of systematic gymnastic work. The hallmarks of genuine collection are lightness, expression, and maintained or improved gaits: a genuinely collected horse is lighter on the forehand than it was in working paces, its movement is more elevated and expressive, and the rhythm and quality of its gaits are preserved or enhanced. Compression, by contrast, is the shortening of the horse's frame through rein pressure that compresses the horse's neck and back without developing the hindquarter engagement that genuine collection requires. A compressed horse may appear to have a higher head carriage and shorter steps — superficial similarities to genuine collection — but is actually heavier on the forehand than in its working paces because the rein pressure that compressed it is substituting for the hindquarter strength that genuine collection develops. The movement quality is typically worse — the gaits lose their elasticity and expression, the back tightens and loses its swing, and the overall impression is of a horse that is restricted rather than one that is carrying with power and lightness.