Horses move into pressure. It's instinct. Sure they shift their bodies away when the rider applies the leg aid, but we trainers teach that response. What you may not realize is the training still permits the instinctive reaction--we've just redirected it. The horse is now stepping toward the rider's leg with the hind leg on the same side--more acceptable than moving their barrel into leg pressure. Oh, and as for the line of travel—that has more to do with where your weight is directed than what you just did with your leg.
But what I really want to talk about is the absence of pressure.
Horses will, if you pay close attention, actually respond to an aid (pressure) when that aid is removed. One of the basic tenets of dressage is that relaxation is required for the horse to move with grace, beauty and optimal athletic results. That means all muscular tension not required for remaining upright and mobile must go. Mental tension produces physical tension, so that has to go too. An aid that doesn't let up—that sustains pressure—is going to make the horse wonder what the heck it is the rider wants. Wondering produces worry, which in turn produces frustration and will cause the trained horse to revert to his instinctive reaction and push back.
Have you ever tried to produce good work while under pressure? It's exhausting, not invigorating. I produce enough of my own internal pressure so my tolerance threshold for external pressure is pretty low. It becomes counter productive. I may not push back in the obvious manner my horse would, but my quality of work suffers. And I get pretty crabby.
Like horses, people have their own optimal pressure tolerance levels. When working with someone (equine or human) it's essential to discover what that level is, and then only use it when necessary. Nobody can tolerate sustained pressure. We all do our best work when it's applied intermittently from the one directing our efforts.
It's a rare thing to have that gift of assessment—the ability to know how much and what kind of pressure is required and then have the guts to release it. Perhaps we can learn if we think, observe, and test our bravery.
Have you ever worked with someone who seems capable of helping you achieve your personal best? Maybe they had to learn how.