I am afraid most backyard horse owners and more than a few people that claim to be 'trainers' actually understand what constitutes 'effective' horse riding and training. Few understand how horse behavior actually works.
An effective trainer (or rider) looks at EVERY ride as a training session. An effective rider goes into every ride with a plan and fully expects the horse to be doing something 'better' at the end of that ride than it did at the end of the last ride, even a relaxing trail ride. To expect less will result in less. Horses seldom exceed our expectations. To get a better outcome than you anticipated is a 'gift' and you shouldn't expect one very often.
I frequently hear people say "But I don't want to learn how to train a horse. I just want to have a pleasant trail ride when I take my horse out."
Well, that is not how a horse thinks. If you are not teaching that horse to do something better than it did before, then it is learning to do something worse. It is a steady down-hill drag until the owner HAS to address the horse's training, has to have someone qualified fix it, turns it into a pasture pet, trades it off or sells the spoiled beast -- frequently to the kill pen. YOU ARE A TRAINER WHETHER YOU WANT TO BE ONE OR NOT!!!
Another mistake that novice owners and 'would-be' ineffective trainers make is thinking that trainers have to be harsh and 'mean'. If a person effectively trains a horse from the beginning, most horses can be trained without anyone ever having to get after them. But, it is NOT always possible to retrain a spoiled horse and get it to stop really bad behavior without getting a lot rougher on them it. It depends on the individual horse, but bad behavior -- particularly dangerous aggressive behavior - has to be stopped and the horse has to know that such behavior has serious consequences. This is a lot different than never letting a horse do bad things to begin with. Again, some owner / rider somewhere is responsible for letting the horse become spoiled. Training a green horse is MUCH different than retraining a spoiled one. It is SOOOO much better to never let them learn bad behavior.
That brings me to some of the questions I read here:
"Why did my horse bite me?"
"Why does my horse try to turn around?"
"Why does my horse stop and refuse to go?"
"Why does my horse lay its ears back at me?"
"Why does my horse put its head down and pulls the reins through my hands?"
"Why does my horse throw its head up?"
"Why is my horse threatening to rear?"
"Why won't my horse back up?" (Substitute trailer load, cross water, stand tied, let me pick up his feet -- the list is endless.)
Horses do (or don't do) all of these things because someone has let them do it before or has let them do little things that led up to doing this bigger thing. They simply do it because they can and have.
The difference between these horse owners and an effective trainer is that trainers notice and interrupt the little things before they become big problems. If a horse pushes against a bit, ever so slightly, the rider should interrupt that push, over-correct it and demand that the horse back off the bit.
If a horse pushes a shoulder out and 'drifts' out even a stride or two, (like toward a gate), that should be instantly countered by demanding a leg yield the opposite direction. If a horse is to be taught to stay between the rider's hands and between the rider's legs, the tiniest deviation has to be corrected, long before the horse runs shoulder first to the gate or a friend or the barn or ????
If a horse tries to turn around, that should be interrupted immediately and the horse should always be turned back the direction it turned from -- never brought all the way around even though that would get it headed back the right direction. That is a 'win' for the horse. To turn back is a win for the rider and an effective correction. To turn back abruptly and rather roughly with a heel or a spur in the ribs would be an even more effective response, especially if this is not the first time the horse has tried this stunt. You want the horse to know that it is unacceptable for it to initiate any move you did not ask for. It is MOST effective to interrupt a behavior as it first begins. This means you 'meet' the horse and fix the problem before most observers can even see what the horse tried to do. A good rider can 'feel' a problem before an observer can see it. This is what is called 'timing and feel'. Good riders and trainers have good timing and feel like having a sixth sense.
Every rider needs to remember that anything you allow a horse to do is what you are training that horse to do -- as surely as though it was the intended training goal.
As for rearing -- that is almost 100% the fault of the rider -- either past or present. Horses rear because they lack forward motion or have a fearful rider that is pulling too much on the reins. Fearful riders cause most rearing. They either lack the skill or nerve to ride a horse confidently forward or they are so fearful that they 'trap' the horse between their pull on the reins and the horse's desire to go forward. A trapped and frequently nervous horse feels that they can only escape by going 'up'. Once they rear and the rider gets off or takes them back to the barn, they have been effectively 'trained' to rear to get out of uncomfortable situations. It quickly begins a spoiled horse's often fatal descent to the killer pen or to permanent turn-out.
Every single thing a horse does that is not exactly what the rider wants or has asked for, should be immediately countered by some action on the rider's part that interrupts or corrects the behavior.
On the other hand, every rider has to be fair. Riders and trainers should only ask a horse to do what it is ready and able to do. Expectations should not exceed the horse's mental or physical ability. If a rider wants reasonable responses, they have to make reasonable requests and have to be ready and able to get the job done.
The rider should not accept less than full and reasonable compliance from the horse. In other words, do not ask a horse to do something that you have not prepared him for and have the time, ability and the full intention of getting done. To fail, tells the horse that it does not have a strong leader that it can trust and that obedience is optional. This is where most trailer loading, water crossing, bathing, spraying, bridling etc problems begin. The more times you fail to accomplish a goal, the less likely the horse is to do anything else you want, either. Pretty darn quickly you have a spoiled horse that only does just what it pleases and could care less what a handler / rider wants it to do.