When we're training a horse, we use both physical and psychological pressures to shape his behavior. You can't neatly separate the influence of these pressures because the horse has physical reactions to psychological pressures and he has psychological reactions to physical pressures. Even so, we're going to try to look at the two separately and how we apply each of them to shape the horse's behavior.
As the horse moves from being a baby green trainee to a horse that can play the upper level games, his understanding of the shape particular physical pressures suggest to him will become more sophisticated. Eventually he learns to understand a whole corridor of physical pressures together into a complex movement much as we put a bunch of individual together to make a sentence that has a much more complex meaning than any of the individual words alone.
In the beginning, however, the baby horse's vocabulary of physical pressures is very limited. If you throw too many physical pressures at him all at once, he will feel attacked. So when we first begin heeding, we spend a lot of time getting the horse to trust us, convincing him that we are a friend and not a predator. We begin to shape the horse's behavior using our own physical actions while he is loose in an arena. We move our primary and secondary lines of influence to put psychological pressure on the horse to get him to move in a particular direction at a particular speed. As we shape his activity, we are careful never to push the horse so far out of his psychological comfort zone that we scare him. We are careful never to raise his excitement level to the point where he loses rhythm or relaxation or his awareness of us.
As the horse's understanding of the game grows, we move alongside him and begin to make the game we want to play a little more complex. We walk, trot, turn, back, stop, change directions. Our main goal is still trust and awareness with rhythm and relaxation. Very gradually we will introduce tack, put someone on his back, get him used to carrying that someone and how their weight affects his balance. The we'll begin to use reins, seat and leg to ask him for the shapes we want. All of these steps mean introducing physical pressures.
There are some physical pressures like the pressure of the girth or the feel of stirrups hanging against his sides we want him to accept and ignore. So you've got to be sure to introduce these pressures slowly and in a way that the horse accepts them and gets used to them without ever feeling that he has to do something to get comfortable.
There's another group of pressures--the ones we apply with reins, leg and seat--which I call methodically applied directional pressures. We want the horse to learn that when he moves away from these pressures in the direction we want, the pressure goes away.
Some people believe that the horse's natural reaction to any pressure is to lean into it. Then to prove their point, they'll poke their fingers into the horse's side. Or they'll point to the fact that a horse's "natural reaction" is to pull back against a tie rope when they feel like their head is trapped. So to train a horse, they say, we have to teach him to unlearn what comes naturally.
These folks have missed a very important difference between either of these situations and a training situation where the horse feels a methodically applied directional pressure. Poking the horse in the ribs or trapping his head is a sudden, startling pressure that raises the horse's excitement level, makes him hold his breath and interrupts the rhythm of his breathing. It scares him out of his psychological comfort zone. His natural fight or flight instincts take over and he either pushes into the pressure or jerks back and tries to make a fast escape.
You need to introduce a directional pressure very slowly and methodically. You show the horse your hand, put it against his side, then put a little pressure there. You slowly increase the pressure and you do not take your hand away until the horse realizes that he is the one that has to remove the pressure. When the horse moves in the direction the pressure is pointing, you have to stop your hand and let him move away from it. You have now taught him the most important lesson he needs to know as his training continues--if he moves in the direction a pressure indicates, the pressure goes away.
When you apply a sudden pressure that the horse does not anticipate, you elevate his excitement level and spoil his understanding. A good trainer methodically applies any new pressure in a way that never surprises the horse. The pressure has to be applied in a way that the horse can remove it by moving in the direction the trainer wants. A methodically applied directional pressure is a solvable problem, not a startling event that causes fight or flight. As a trainer what you're trying to do is develop the habit in the horse of responding to or resolving these directional pressures in the same way every time.
The release of a physical pressure is very important to the horse's understanding. They need to trust, for example, that if they turn their head left when the pressure increases on the left side of their mouth, the pressure will go away.
The same thing holds if you ask a horse to back. I see people start fighting with a horse to back and when he backs up they keep fighting with him to keep more back going on. What they should do is ride back a stride then soften everything for one stride to show the horse he did everything right. Then very quietly apply the same set of pressures for another back stride and reward for that one and so on.
Pressures have to be shaped to match what we're trying to accomplish. You'll see some people flapping and slapping their horse's sides with their legs to keep the horse at a canter or gallop or other people who just clamp their legs on and never let go. Then another time they squeeze the horse's sides and nothing happens. Well that's because nothing happened when the horse tried to respond to the their slapping or clamping and the pressure stayed there anyway. These people are not shaping their pressures in a horse logical way.
Watching good training is boring unless you understand what's going on.