At some point it its training, something will startle or frighten an energetic, red blooded baby horse and he will rear or pull back or run sideways while the trainer is leading him. Or he may jump around just because he's young and he's feeling good. Or maybe he's challenging his trainer like he would another horse in the herd just to see who's who in the pecking order.
These things are actually the trainer's fault because they allowed the animal's attention to wander. Then an awful lot of trainers make a second mistake. To get the horse's attention back, they jerk the shank or yank the horse sideways or pop him with the end of the lead rope or they yell at him.
This is the "biggest, baddest wins" school of horse training. This method sometimes looks like it works. If the trainer really is the biggest, baddest one, they may get the horse to freeze and hesitate before they startle or rear or pull back the next time. But the horse hasn't really learned anything except that when they're frightened or startled, they're going to get attacked so they better watch out. That's not a lesson you can build on to teach the horse anything else.
The trainer intends these jerking or pulling or popping pressures as punishment for the horse's "disobedience." They think if the consequences of a particular behavior are bad enough, the horse will avoid that behavior. But it doesn't work that way. The horse feels shanking, jerking, yelling, or popping as an attack. So instead of shaping the behavior the trainer really wants, these things just accelerate the behavior they were trying to correct.
Most people are scared when a horse rears up. Their first reaction is to jerk on the lead rope or get out in front of the horse and pull on it. Pulling down on a horse's head gives the horse the feeling of being trapped. The fastest way to put a rearing horse over backwards is to keep pulling on his head because his natural tendency is to fight back against the pressure. Just the same, if you get out in front of a horse that's running back and start pulling on his head, the horse will just go backwards faster. You'll see horses running backwards with someone running right in front of them holding on to the rope and jerking. To the horse, this is a head on attack that just drives him back more. If it's a horse that's challenging you or unhappy for some reason and you get in front of him, he can get you with his left or right front foot or with his teeth.
The only really safe place to be around a horse is close enough to it so that it can't get any swing going with anything. That means at and right against the shoulder. When you work with a horse, you always work from the shoulder back and from the shoulder forward as you get to know the horse. When a horse rears as you are walking beside it, you want to stay as close to the shoulder as possible. The front feet are what will hurt you and if you can stay against the shoulder, there is no way the front feet, back feet, or teeth can get you. If you need to, grab a chunk of mane and pull yourself against the shoulder. You give the horse all the lead line it needs to go up.
The best way to deal with rearing or pulling is not to let them get started in the first place. You do that by keeping your attention on the horse and the horse's attention on you at all times. Every stride. Nobody's perfect, however. So if the horse does startle or pull back or rear, you just go about your business and put him right back to work. Don't attack or punish the horse for "being disobedient." Remember, there is no such thing as a disobedience if you're not directing the horse. That means you have to be telling the horse what TO DO and what NOT TO DO. Pulling or rearing or jumping sideways may be a lapse of obedience but when they happen, you simply interrupt them with instructions of what to BE doing. No punishment. No fight. No fuss.
Your primary objective in any training session whether you're working on the ground or from the saddle is rhythm and relaxation. What the horse needs is steady, physical work at a mental level that you have created which is alert enough and excited enough to pay attention to you but not frightened and not tense. He's just looking to have a good time, and that's what we're trying to teach him to do--how to have a good time playing our game. If he gets startled or frightened, you want him to come to you as the safe place to be. You want to be a person he can trust for some direction to get him past whatever is frightening or startling.
When you're working with a horse, pay attention to his ears because they'll tell you where his attention is and whether he's relaxed. Whether you're walking alongside him or up on his back, you want one or both of those ears swiveled in your direction to let you know you have his attention. If you don't, put him to work with some heeding or change what you're asking for under saddle just a little until he gives his attention back to you.