Martial arts include skills relating to self defense, fighting, and sport; however, each of these has different goals and different constraints. Even within these areas, there are different focuses that are still easily visible in the differing development of many arts. Take, for example, the differing characteristics of Bagua, Xingyi, Chen Style Tai Chi, Yang Style Tai Chi, or Wu/Hao Style Tai Chi, each of which clearly reflects the circumstances around its origins. Who is your likely opponent? What will you and he be wearing? What do you want to do to him or her? Where will you face him or her?
If you prepare for everything, you will not be particularly good at anything. MMA is a sport that is well adapted to some martial arts, but not to others, even many external martial arts. Almost all the top MMA fighters come out of only half a dozen martial arts/sports or just generically train in the pool of techniques from those arts. In my view, this reflects poorly on neither MMA nor on those other arts. It would be like asking why Karate fighters and their techniques are not better represented among professional boxers. They are simply different things.
As for how to fight with internal arts, I would say that each traditional system has its own curriculum. Follow through diligently until the end and you will have your answer. In our case, I would summarize our path by saying:
1. Do form and drills to learn to manage your own energy.
2. Do fixed-step push hands to begin to learn to stick and manage the joint energy of you and your opponent.
3. Do moving-step push hands to learn how to move while establishing and managing your joint energy.
If you do all these things, you will learn how to control your opponent's energy. If you can do so, what can your opponent do to you? What can you not do to your opponent?
There is a great deal of content along this path, and different people focus on different aspects. Some styles or teachers promote shorter paths or shortcuts, but I do not have know of any that fit our system. We use form to arrive at no-form. We focus on thought, so that we do not need to think later. The range of techniques and how they combine is limitless. "Become familiar with the movements, then come to understand the energy in them, then you will have marvelous insight to do what you want."
There is no way to describe such a thing in a few words, but let me quote Sunzi
, as one of my Tai Chi teachers often has. What applies to a clash of armies is what is to apply between you and your opponent.
Scheme so as to discover his plans and the likelihood of their success. Rouse him, and learn the principle of his activity or inactivity. Force him to reveal himself, so as to find out his vulnerable spots. Carefully compare the opposing army with your own, so that you may know where strength is superabundant and where it is deficient. In making tactical dispositions, the highest pitch you can attain is to conceal them; conceal your dispositions, and you will be safe from the prying of the subtlest spies, from the machinations of the wisest brains. How victory may be produced for them out of the enemy's own tactics - that is what the multitude cannot comprehend. All men can see the tactics whereby I conquer, but what none can see is the strategy out of which victory is evolved. Do not repeat the tactics which have gained you one victory, but let your methods be regulated by the infinite variety of circumstances.
Military tactics are like unto water; for water in its natural course runs away from high places and hastens downwards. So in war, the way is to avoid what is strong and to strike at what is weak. Water shapes its course according to the nature of the ground over which it flows; the soldier works out his victory in relation to the foe whom he is facing. Therefore, just as water retains no constant shape, so in warfare there are no constant conditions. He who can modify his tactics in relation to his opponent and thereby succeed in winning, may be called a heaven-born captain. The five elements (water, fire, wood, metal, earth) are not always equally predominant; the four seasons make way for each other in turn. There are short days and long; the moon has its periods of waning and waxing.