Ok, lets see if I can do this without rubbing anyone too terribly wrong. First of all, what is your goal with this horse? Many people say that they just want their horse to listen to them, they want their horse to be obedient, do what they ask without question, total loyalty and devotion. Some people say they just want their horse to love them and/or respect them. Some want top performance, they want an athletic machine that loves to show off its stuff. Ultimately, I don't think that I would be horribly wrong if I said that most people want all of those things. But here is a question for you, what does your horse want?
After that question, many people would sing the praises of NH, which aren't entirely wrong, but I still have some questions. There are many things that we can get our horses to do out of a series of pressures and releases. Once the horse recognizes a person as a herd leader, then they will do all sorts of things. Here is the catch, a horse has to be about 90% lame before showing signs of it. It will contort its body in all sorts of ways just to get away from the pressure. So..... Just how healthy is it for all that disengagement when the horses axial rotation is so limited? Isn't our ultimate goal engagement? It turns into a pretty rapidly learned behavior, and the horse will compromise itself considerably just to do what is asked of it. Here is another question, why are we going to disable the horses power house by taking straightness out of the body? I have heard many times in the NH world "don't let this horse go straight, that will give the horse the power to do this, that or the other thing" Um, yeah, straightness gives the horse the power to do all sorts of things, so why are we going to disable that rather than learn how to control it properly? Or how about "when they buck, disengage their hind end" it does stop the bucking, and I would do it if I happened to be on a horse that was bucking, but if I had done my job properly, that horse wouldn't have the need to buck in the first place, it is only a temporary solution. NH works wonders on the mind, but many times the physical needs of the horse are left behind. It makes it easy for people to follow a program to get the results that they want, and they tell people that it is what the horse wants too. They keep people moving through the program with things such as "your horse will get bored if you keep doing the same thing". I'm sorry, but I do use my horses for work, every now and then they may have to stand around for a little bit, I have never seen one get bored. People get bored, horses in a sound frame of mind have no problem standing still as long as they feel safe and secure. Or my favorite, buy this contraption, your horse will love you for it, even though in reality, the horse is just relying completely on that because the rest of its body is incapable of holding itself properly.
Don't worry, I won't forget about the other side here either. How about those performance horses? I have worked with some horses with price tags higher than my house, and those things were as obnoxious as they could be. Their bodies are in phenomenal shape, but many of them aren't even allowed to interact with herd for fear that they might get hurt. They also learn physical patterns, often times through some type of direct force, to get them to keep their head a certain way, jump this, move legs like that, but I know many of these people that don't even interact with their horses out of the saddle. IMO, that is not the ideal also.
So here is my challenge to you, let your ultimate goal be the horses health. Every horse has a correct way of going, a biomechanical soundness that gives the horse optimal power, and you would be surprised that once you find what makes the horse healthy, the behavior problems seem to melt away. Does the mind need to be addressd? Yes, of course. I am not at all against using NH techniques to help find that mind, but I am against excessive use of this just to give the human the warm fuzzies. I had a horse come in less than a week ago with problems similar to what spirit horse describes. By day 4, the owners small children could handle him again. I never touched him harder than a light tap. Now, in less than a week, I'm moving on to making his body stronger, since this is actually what was originally causing the problems with his resentment towards his owners. Many horses issues are originally caused in the body, so why does NH spend so much time working around those issues? Many horse owners are led to believe that their horses problems are behavioral, preventative and corrective riding isn't something that most people even think of.
So, my advice to the OP, let your ultimate goal be the overall well being of your horse. (sounds simple, right?) As a yearling, you do have time to work on the mind, but make sure that your work on the mind is not harming the body, especially such a young and fragile one. It is as much of a physical and mental necessity on your part as on the horses. This will require a lot more of your mind than watching DVDs, it will require you to watch your horse, understand its movement, understand its mental state, and move everything towards optimal. I personally don't care if my horse can push a giant ball or jump a 5' fence, I am actually more impressed by a horse that can move willingly with ease, grace, straightness, and impulsion. Good luck with whatever you decide to do.