The first is physical, and this is the essence of combat: applying force from your body or tool to the threat or threats. Avoiding similar damage coming at you. There is a lot here, enough to make thousands of techniques and to justify the hours and years of training. Sometimes I seem dismissive on this aspect. Partially it's because once you have been doing it for a long time, it seems pretty simple. Power generation, targeting, power conservation and the infinite quest for the most efficient motion. Take some comfort in that if you want, but that's not the real reason I don't spend a lot of time writing or thinking about this. The real reason is that I have rarely seen anyone with any training who was crushed in an assault because of a lack of physical skills. Almost all simply choked. They knew what to do, they couldn't make themselves do it. So the physical side of it, in my opinion, is a critical skill to success, but does nothing to prevent catastrophic failure. That comes from elsewhere.
The second is the cognitive stuff. Strategy, tactics, evaluation and planning. The ability to see what is going on and make a decision. Right here, the infinite ways to see and prioritize these possibilities are the reason for the huge variety of combative systems. Do damage or avoid damage? Attack the mind or the body? Minimal harm or maximal harm? Break bones or disrupt balance? Weapons common or rare? Expect multiple opponents or duels? Ambushes or matches? In each of these pairs, the one you emphasize (no one discounts one of them entirely, though people sometimes argue as if they do) will drive how you move and what you teach.
There is a big potential for failure here IF the students are led to believe that the strategy and system they are learning is perfect, or even good, for all situations. Tactics and movements from an unarmed duel aren't the same as an armored medieval battlefield or an ambush from behind at a urinal.
But it's an easy fix, to an extent (and this is not a guarantee of success, nothing is): From day one students are taught to keep their eyes open, don't count on anything, and be ready to adapt. That's my personal definition of ju in a nutshell.
Third is the emotional level, and there are really two of these.
The first is what all the people who like to talk about the warrior identity call 'spirit'. This is the internal aspect of emotion: How much do you feel fear and how do you deal with it? More important, in my experience, is the question of security. Can you act when you can't begin to predict the outcome? Maybe it's a level of faith, maybe confidence, maybe ignorance and maybe those are all aspects of the same thing. Is your instinct when you are pressed or scared or someone screams to deal with it yourself? Or do you look around for someone else to deal with it? Or pretend it's not happening? People have been brutally beaten and some have probably died curled into a little ball hoping mommy or the cops or the cavalry will come save them.
This is the source of a lot of catastrophic failure, and the source is strictly internal.
I haven't seen a training system that grants what you need for this. I have seen it grow in individuals over time when those individuals were exposed to other people they respect who have it. And I've seen it fail to grow. Still speculating on the reasons, trying to sort out why and who. (I have seen one system that tests for it in a single minute. That's cool, but the injury rate is pretty high.)
The fourth is another emotional level- the social screaming monkey level. The psuedo-cognitive level that always wants to know 'what does this mean to me?' It is the social mind that wants to put everything in a social context- does this person trying to kill me hate me? Did I do something to deserve this? Why is this happening to me?
The thing about this is that tries to deal with a violent situation from the rules and point of view of a regular world that doesn't countenance violence. It is just like trying to cling to the plane after you have already jumped. It's too late for that. The monkey mind insists on trying to analyze a social solution to what has become a physical problem. Right here is where a lot of the freezing and the catastrophic failures happen.
This is another level of 'letting go'- some people don't realize when they have stepped through the looking glass into a new world. Others can't let themselves work under the rules of the new world. Some is conditioning, which goes deeper in some people than others.* For some it may be wiring- and this can go a couple of ways. Not just physical fear, but fear of the unknown are different in different people. The balance of whether physical damage or death is more demotivating than social embarrassment and humiliation is another. (Point, logically it would seem that death was a bigger fear than social concerns. If that were true, armies would dissolve and teen age boys wouldn't do all the stupid things that we remember so fondly. No one would smoke or use drugs or alcohol. Fear of death isn't as big a motivator as it logically should be. Just an observation, not a position.)
These things all contribute to whether someone can let go of their civilization, slip the leash and actually do what they have trained, physically, to do.
Not only is there a lot of catastrophic failure stemming from this, but there is a lot of denial on the subject. This is one of those things that being told your mind and body will change, that you might freeze, that you may not be able to make yourself do your patented fight ending move gets shrugged off. Or instructors parrot the words, but in this case the words are not the music.
So far, and I like to think it's been pretty successful, I describe as well as I can what to expect and how I have broken the freeze. Just get them to recognize that they have crossed that border. Let them know that the rules are different there.** This is one of the places where I do give a talisman to selected people. Here it is: "Do you have the will to survive? Are you a good person?" If you have both those things down to your soul, you will do the right thing, you'll do what you need to do and no more. Let your body run with it. Trust yourself."
*Everyone who teaches women to fight needs to read this article. Originally written for heavy fighting in the SCA it was a very valuable tool for teaching not just martial arts students, but also cops. Tobi went even farther in the book, "The Armored Rose." Out of print, but well worth the price.
** It's not a world of savagery and blood, either, at least not for me. Professionals are required to do the right thing in there and by and large we do. There are rules in violence, they just aren't the same ones.