To properly or fully benefit from Tai Chi it is important that we understand the breathing technique involved - and coordinate our movements with these breaths. The breathing technique intrinsic to this is called "Diaphragmatic Breathing". This is sometimes more popularly called "Circular Breathing" because of certain visualizations that I describe later, however the latter term actually refers to a technique used by players of some wind instruments (such as the Australian didgeridoo) to produce a continuous tone without interruption. The "Circle Breathing" in Tai Chi is more ... 'in the mind' and a thing of mental discipline and focus as opposed to a thing that requires any special or exceptional physical ability.

The graceful movements of Tai Chi Forms may, with little alteration can function as a martial art fighting form of Tai Chi Chuan - and in this respect - all of the postures within the Form may be categorised as either 'offensive' or 'defensive'. Accordingly, all defensive postures (or 'moves') are performed on an in breath - and all offensive postures/moves are made on an out breath. Furthermore, a defensive move is classified as a "Yin" and an offensive as "Yang".
So the theory is: Defense on the Yin or In breath and offense on the Out or Yang breath.

The natural, instinctive way to add power to any intended 'out' or away or push movement is to first set yourself, breathe in and then … puuuussshhhhhh on the out breath. We do this all of the time to a greater or lesser degree in our everyday activities, from opening a stuck door through to pushing your broken-down car. When we are not doing this, we are, to a greater or lesser degree, doing the opposite! For instance, you see a bird flying towards you and you fear that it may be a little low and it will have your hat off. What do you do? You breathe in and turn side-on by bending and sinking at the knees a little. Doing the dishes, you drop a cup. What do you do when it hits the floor and shatters? You take a sharp in breath as you jump back a little and raise your hands towards your face, subconsciously protecting your eyes from shards. Sometimes, because of the same primitive instinct, we might even shut our eyes or squeeze our lips together. Our ancestors did the same when snakes and such like creatures spat who knows what at them!