‘Balance’ in taiji
One of the aspect of taiji is to allow ‘freedom of movement’, so that the body never inhibits itself.
‘Freedom of movement’
In order to exist, a solar system must work in harmony. If one planet in the solar system were to go out of alignment, the system would become unstable.
If you see your 
body as a solar system, the torso being the sun at the centre of the body, and your hands and feet and head being individual planets within that system, those planets always need to ‘balance’ one other.
Left/Right; Front/Back; Up/Down
‘Ease of movement’ is produced first of all by relaxation. Any tension means that the body is fighting itself and therefore restricting movement … it is ‘inhibiting’ itself.
Ease of movement is also created by the interaction not only of the four limbs, but also of the upper and lower body. Horizontally, it is about using one side of the body to help the other side, and on the vertical aspect, it is the upper half of the body helping the lower half. In tai chi, movement never happens with only one unit of the body (e.g. one hand or leg), it is always reflected throughout (as in Newton’s 3rd law: ‘For every action there is an equal and opposite reaction’).
As an example of vertical balance, you only have to think of a tight-rope walker to get the idea – he uses a long pole that stretches to either side of him to increase his ability to balance (the pole distributes mass away from the vertical axis which is the pivot point, which increases inertia). The point is that left and right have to be balanced for this to work.
Auto-balancing
Physiologically, when damaged, the body can balance itself automatically over a period of time when left alone.
An example of this is that if you have ever damaged your lower back causing the lumbar muscles to go into spasm, you will know that your neck can be affected. After a while, the neck gets used to the new state of affairs and adjusts itself. You go to the osteopath who sorts out the lumbar problem, and your neck gets sore again, eventually recovering as it again adjusts itself.
Open/Close
Returning to the solar system analogy, the ‘magnetic’ push/pull of the planets, which holds them in their orbits, is comparable to the physical action of breathing.
At the most basic level, an in-breath has to become an out-breath; you cannot go on breathing in forever. This translates as ‘open/close’ (Kai/He); an ‘open’ must become a ‘close’ – you cannot continue opening forever, nor can you ‘open’ without having closed first…
E.g. it’s impossible to punch with your arms already outstretched; you have to draw them inwards first.
Neither can you have only one side of the body ‘opening’ because the body then becomes unbalanced.
E.g. when breathing in, it is obvious that the chest expands; but the back must also expand.
It is more obvious that when breathing in, not only does the left side of the ribcage expand, but the right side also expands.
Two-Person Tai Chi
Two partners, working together in tai chi, e.g. in Pushing Hands, are like two solar systems interacting; for mutual existence, one needs to sense the actions of the other and respond accordingly. It is this interaction between the two partners that produces the softness of tai chi.
Natural rules of movement
The odd thing is that, in the everyday actions of our day-to-day lives, we follow these ‘rules of movement’ 90% of the time. But when learning tai chi and other forms of movement, we start thinking, and all of a sudden our natural movement habits seem to vanish! So we have to re-learn the principles of what we would normally do without thinking. It’s also worth pointing out that when we don’t follow these principles in our daily lives, through distraction, stress, hurrying, or laziness, we often injure ourselves (e.g. picking up a heavy object from the floor without balancing the front and back of the body).
Daily use
A simple example of our daily use of the body compared to our ‘learnt’ use of the body:
If you wanted to pull a very heavy object, for example a wooden table, across a room, you might begin with one foot in front of you and one behind, you’d take hold of the edge or leg of the table, and as soon as you started the pulling action, you wouldn’t leave your elbows stuck out to the side, (you’d drop them), nor would you leave your bottom sticking out, (you’d tuck it in). If the table were extremely heavy, you would additionally engage your abdominal muscles, possibly sucking them in…
As you dragged the table across the room, it would start to feel as though you were using your back to help pull it; and that’s exactly what you’d be doing, you’d be balancing the front leg (that you’re using for pushing) with your back.
Learnt use
However, when it comes to learning tai chi, most people just move their weight backwards (from the front leg to the back leg), do nothing with the pelvis, don’t even drop the elbows, and just pull their arms back (bending the elbows) as though the arms are a couple of additions to the body that just happen to be in front of you. All they see is the superficial movement of a body moving backwards from the front leg to the back leg, whilst withdrawing the arms.
So in many ways, doing a solo form of tai chi is much harder than doing tai chi with a partner where contact is involved. In solo tai chi, it’s harder to feel that you are pulling or pushing as there is nothing to pull and push. Your body therefore behaves differently. It’s almost easier to perform movement efficiently when working with a partner, because often (though not always), the body will balance itself automatically, and will cease to inhibit itself.
Which is why, when performed correctly with the intention involved, solo tai chi is often referred to as ‘shadow boxing’.
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