J.L. -- There is a big difference between having the horse bow (which he does himself) vs. a rider pulling the head into an upside-down position as in "rollkur". The horse that volunteers an action will very rarely hurt himself thereby. No rider, no matter how expert they think they are, is as aware of where the horse is in terms of muscle or ligament strain as the horse himself is. In human physiotherapy, it is considered normal and courteous of the therapist to continually ask the patient "am I going too far? Does this hurt in a bad way?". It is a far more subtle matter to ask and receive this information from a horse, and most competitors could not and would not ask, or if they do, they don't much care about the answer: hence the damage they regularly create.
But you have no intention of creating any damage, and your horse will certainly benefit from bowing, no matter how deep, just so long as he volunteers it and is not compelled. -- Dr. Deb