The hard-mouthed horse is a topic that is common, and very relevant, in training or retraining horses. It is something that has been claimed to be fixable through countless flexions of the neck and jaw, a change of bits, nosebands, and various techniques.
What is a hard-mouthed horse exactly?
To be honest, I believe there are several different ways to reference a hard-mouthed horse. Some are defined by the inability to flex the horse in whatever direction is desired, others self-diagnosed as such while they are running away with their rider – refusing to stop or slow down.
Are hard-mouthed horses created by the rider and training, or is it natural to some horses and not others? Both. Horses can learn to use it as a method of self-defense, others it is their natural reaction to communication with the reins that has never been worked through properly. Just like some people enjoy having their feet massaged, others it drives them crazy because they are ticklish and so they tense up the moment they believe you are going to touch their feet… same principle.
So what, then, is the key to solving the hard-mouthed horse?
It is a combination… none of which involve directly flexions or any auxillary hardware (so you can just set those draw reins down and step away slowly…). The first is fixing ourselves – always the first step. Chances are more than great (probably close to 900% probability) that any rider having issues with a hard mouthed horse is pulling in some way shape or form on the reins – either to direct and/or control the horse, or (and maybe coupled with) using the reins for supporting their balance in the saddle. The vast majority of riders struggle with both of these issues because a) pulling is a natural response for our body when we are in a state of anything but complete and total relaxation and self-awareness; b) having an independently secure and balanced seat in the saddle is not standardly taught in a fully functional fashion for most riders.
Pulling on the reins will elicit resistance from the horse. Even if the horse is soft in the mouth, the resistance will form in some other area of the body in order to accomodate the pulling of the reins by the rider. Pulling on the rein as well is a hard thing to describe in it’s entirety, because pulling doesn’t have to be forceful, which is what most of us picture when we think pulling, but any backward action of the rider’s hand can be interpreted as pulling by the horse.
And in regards to a rider’s seat, without having independence of the seat (not relying upon the reins, legs or stirrups to maintain security and being able to act separately from the legs and hands both unilaterally and bilaterally), everything we do with the rest of our aids is compromised because it may be conflicting due to the mixed use of balance and aids. This is why I put such strong emphasis on the importance of seat training early on for all riders, because with a secure seat the rest of the aids come easily, and it often resolves many other issues that riders are currently or may potentially face with the horse.
Back to resolving a hard-mouthed horse…
Once our own issues have been resolved to avoid conflicts and mixed signals to the horse, we can move on to addressing the issue straight on if it is still present. We usually interact with the horse in a sort of monologue – we tell the horse what we want and he either does it or doesn’t do it, which may or may not result in more or less telling. Rarely do we listen to the horse as thoroughly as we expect him to listen to us, not even a fraction of such. So the second part is learning how to tap into a conversation with the horse that is equal rather than unilateral… which I will go deeper into in another post. 
