The simple laws of physics state that for every force or action, there is an equal and opposite force or reaction. This is a universal law that applies to everything.
In Horsemanship, or when it applies to people or animals that think, it can become amplified and accelerated into perpetual motion. This happens because one applies a force, the other applies a force to bring balance. In the case of thought, whether it be ego, inconcideration, or any form of inhibition, an extra force comes to gain an upper hand. This in turn begets a force to establish balance again. The cycle can intensify in this way. This is how we get the "HARD MOUTHED HORSE" Many other problems like stiffness, bracing, refusing, escaping, and many more come from this same basic natural law.
If thought is the energy or the force that is in play, and they are respectful and meaningful, then the responding force will be one that maintains balance and harmony. This will keep all actions and reactions civil. It maintains a balanced mind in the Horse and the Rider.
Sounds easy right? The key is to do more with thought and energy and less with restriction through Reins and legs. You can use your Reins and legs to create thought, but that alone isn't enough. You have to communicate and maintain thought while helping your horse do the same.
This is where I differ from other trainers. Yes, all trainers work on some form of cue development process. The better you understand this the better things get. You can get a horse to a do a lot of things through cues. But the real softness and suttleness comes from them being able to think with you. Not responding because they know what's next, but responding precisely at the moment it needs to happen. Always on time and never early or late.
I can't write about how this works, you have to see it. And even then you barely get anything from it until you start working to develop it.