What is the difference between a horse behind the vertical and one on the bit?

The distinction between a horse that is genuinely on the bit and one that is behind the vertical is one of the most important technical distinctions in dressage, and confusing the two produces training that creates the appearance of correct work while actually developing the horse in a direction that limits its future progress and compromises its welfare. A horse that is genuinely on the bit has its face at or very slightly in front of the vertical — the imaginary line from the horse's forehead to the ground is perpendicular to the ground or the nose is slightly ahead of it — and this position results from genuine engagement of the hindquarters, a swinging back, and the horse reaching forward to seek the bit with a soft, yielding jaw. The contact in this state is elastic and genuinely two-way, with the horse's energy reaching the rider's hand and the rider's influences reaching the horse's hindquarters. A horse that is behind the vertical has its nose tucked behind the vertical line, with the face pointing backward rather than perpendicular — a position that in most cases results from excessive rein pressure or from training that has taught the horse to avoid the contact by drawing its head inward. The contact in this state may feel light or even nonexistent because the horse is not genuinely seeking the bit but avoiding it, and the rider's rein influences cannot reach the hindquarters because the horse's neck is compressed and the energy chain is broken. The horse behind the vertical cannot work through its back in the genuine way that on-the-bit describes, and movements performed in this position — however technically correct their footfall — lack the genuine expression and suppleness that the judges evaluate when they award high scores. Rollkur, the extreme behind-the-vertical position used by some international trainers, represents the most controversial expression of this training approach and has been subject to significant welfare criticism and FEI regulatory attention.