I often divide up what I am teaching into three areas when people ask me about it: movement, interaction and fighting.
Movement means not just moving, but moving in the most natural way possible. You really can't move incorrectly actually, but you can move and not be in the right state of mind for movement or you can move inefficiently for what you are trying to accomplish.
Most people move quite well in many areas of their lives. It is not unusual to be completely awesome at putting on a sock or tying a shoe. Its usually the bigger movements where we run into problems.
One area where you can run into problems are simple whole-body movements like walking. If your intent is directed inward and you are wrapped up in your head, you will probably lean forward. This results in the body being out of balance. If your body is out of balance, you have to use muscles to compensate. Using these muscles all day leads to soreness and eventually chronic problems.
Some people have suffered stresses in life to the point where they can't let go at all when they move so they keep whole areas of the body locked up.
The body can only really relax and move naturally when emotions are allowed to flow. So some people have been given a reason to try to stop themselves from experiencing and/or expressing certain or all emotions. This will cause the body to lock up in reflection of the constricted emotion.
Some people can move well when they are alone but experience serious problems when around people or when doing specific activities. If you are worried about what people think or how you look, you can't move naturally (or you could say the fact that your attention is in the wrong place will cause interruption to natural movement). If you don't think you can sing and are worried that your singing (or any other activity) is inadequate, you will probably clench up in preparation for beginning to sing.
So I try to help people overcome these kind of issues and continue to work on my own. Some of the problems people have in this area tend to be pretty universal while others are specific to the individual.
These issues are also extremely "deep" in a person. What I mean is that it isn't a matter of showing or telling a person how they should move. That doesn't work. Their movements are tied up with their identity, their past, their parents, their culture, their everything. You have to go back to very simple slow movements and relearn things you already know how to do. Along the way, you also have to confront a lot of personal emotional issues. I find that I have to use a delicate touch (difficult for me) when dealing with these areas. It is no accident that there are whole schools of psychotherapy dealing with movement and emotional health.
All things being equal I would rather not get into the way people move and control their bodies and just teach straight fighting. It can be frustrating and messy stuff. So why do it?
Because you can't get really good at anything if you have fundamental movement issues and what's worse, the activity won't be fun on a fundamental level. If I teach wing chun without addressing these issues, people will do it, get better, then get a little better, then quit without much to show for it. If I teach 100 people, I will get a couple of people who already move naturally and they will learn to fight very well. Another couple of people will stumble upon the right way to move on their own or through some eureka moment. What about all the rest? Those people deserve to be great at something too, and I cannot in good conscious take their money and teach them just wing chun, when I know that they are not ready yet to make use of that fighting strategy.