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. 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. 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.