If you were to ask me to sum up what Tai-Chi does in one easily digestible sentence then my answer would be "Learning how to make friends with gravity". Both as a physical discipline and as a martial art Tai Chi teaches you how to understand how first your body and later someone else's work in relation to the pull of gravity.
If you imagine a set of plates stacked up neatly, one on top of the other then the structure they represent is structurally quite strong and can absorb a large amount of downward pressure. However if we took those plates and moved just one or two so that they were out of alignment with the others, then suddenly the structure becomes much more unstable.
The body can be seen in much the same light, our bodies are designed to work with gravity, each piece neatly balanced on top of the other so that the weight of one section rests on the one below and all ultimately rest on the support of the ground, if you look at the way a small child moves and learns to walk then you can see this process of alignment in action. However as we age and the emotional and physical strains of life take their toll our body becomes misaligned. The causes can be many, physical through injury, overuse or simply through long sedentary periods in front of a desk, or they can be emotional for instance the defensive withdrawn body language of depression or the defiant chest out of aggression.
None of us can ever be perfect and in this sense and our bodies will always present some form of misalignment however Tai-Chi aims to gently re-educate the body, teaching you how to move and act to allow gravity to work for you instead of against you and thereby minimise it's detrimental effects.
Tai-Chi initially uses sequences of slow co-ordinated movement exercises which allow us to both strengthen the body and pay close attention to our movement so we can begin to generate strength through our alignment, once we can move more naturally following these principles then you learn how to apply them at greater speed and (if you so wish) in a martial manner. The main sequence of these moves is known as The Form, it's the thing that most people commonly recognise as Tai-Chi from seeing film of it in various guises. Each school of Tai-Chi has it's own form(s) and each are performed in slightly different ways, however all follow the same set of principles, let the pelvis sink, the head rise and allow the weight of the body to relax down into the ground.
The youtube clip below is of Cheng Man-Ch'ing (the founder of the style of Tai-Chi I teach, see below for brief history) moving through the form.
If you watch closely you may notice that he is keeping his body aligned with gravity at all times and moving his body as a whole unit, so no one part of his body is moving independently from the other. Watching him go through the movements of the form it seems quite effortless, however he has practised many years to get that good and most people are often surprised in the amount of exertion it takes to do the form properly. However it is not my wish to swamp people with complexity I aim to break the moves down and introduce people to the concepts gently so that when you come to learn The Form it's moves and principles are familiar.