To have a horse on the bit is not to bring the horses head down. It means many other things.
I believe that no other term in dressage has been the object of so many articles, videos, books and, of course controversy, as on the bit. In this short commentary, I want to explain what does it mean, what other terms are used by professionals that also reflect the concept and what are the conditions necessary to have your horse on the bit.
The FEI Rules state that a horse is said to be on the bit when the neck is more or less raised and arched according to the stage of training and the extension or collection of the pace, accepting the bridle with a light soft contact and submissiveness throughout. The head should remain in a steady position, as a rule slightly in front of the vertical, with a supple poll as the highest point of the neck, and no resistance should be offered to the rider.
Some experts and authors prefer the term on the aids, in order to emphasize that it is not only a head position but rather that the horse is in all 3 aids: seat, legs, and hands.
Others prefer the term acceptance of the bridle, a term also used in the FEI Rules, that add that the horse must move with submissiveness throughout and without any tension or resistance.