Regarding the use of Tai Chi Chuan as a martial art, this system is built up in a manner, where it's trained and appears as a soft martial art. In Tai Chi everything depends upon yielding. At the beginning (at least the first 3-5 years) much emphasis must be put on learning how to "invest in loss", as the Tai Chi Classics put it.
You could surely say, that you try to intend a specific kind of relaxation in mind and body. An almost alchemical process, that takes many years to fulfill, and actually is rather seldom unfolded in a human life, especially in these hysterically busy times. Understanding that this process is possible, is very difficult for must of us. This is mostly because we all put up mental limitations upon ourselves. If you cannot get the principles of Tai Chi Chuan, cannot make themsucceed or work according to the Classics, you can be dead certain, that it's yourself, consciously or that blocks the road to understanding.
The irony in this situation is obvious, and typical to much human development. Of course we are here seeing our own superb arrogance - and it's not that easy to deal with - is it? Maybe here we can introduce a kind of "Psychology of Shadow Boxing" into our own inner exalted apartments? It becomes even funnier, as we consider, that we use much more energy (which means quite some) to keep ourselves on our present narrow and energy demanding level, than we actually need to use to develop, unfold and blossom. Tai Chi Chuan is easy and simple - but it takes quite a long road to reach that point.
If you have ever tried being pushed during training by someone who has been training correctly, you will know, that the push you received was soft and threw you far away - actually a very pleasant and paradoxical experience. Often the push will be accompanied by a sense of energy or pressure, which - if it's expressed fully - can be most damaging to the inner organs. The energy you work with here is called Jing, and is Qi in its cultivated form. You are supposed to train forth Jing in your body, and in this process you use the Mind or Awareness. A genuine Tai Chi-master will of course know how to use Jing correctly.
The only thing you can use it for, apart from your own personal and powerful journey, is to teach others about this wonderful Qi that fill the Universe - how to get in contact with it, and how it enlightens the human Spirit and Body. Many of the Classics are poems, which talk about this level.