It can be dangerous to try to pigeonhole dressage techniques into "classical" and "not classical" or "modern". Good training is good training, and bad training is bad training. Both existed then and both exist now.
Good dressage seeks to work with the horse and cultivate his natural beauty and movement while teaching him to be a willing and responsive partner. Bad dressage forces the appearance of that final product, often using short-cut techniques or equipment to bypass portions of the training scale. To the trained eye (not saying my eye is that trained at all here...
) horses trained this way have visible earmarks of the skipped steps, such as a hollow back, face behind the vertical, uneven muscling - esp. Of the neck, poor rhythm/impurity of gaits, tension and stress.
Check out this website: ::: Sustainable Dressage - - Welcome to my Site about Sustainable Dressage! :::
Some of the articles get rather technical, but this is an excellent resource for understanding the theory of good dressage and training in general.