Yin and Yang
The concept of Yin and Yang is so basic to Tai Chi that instructors often forget to explain it. I think that part of the reason lies in how common it is to see the Yin Yang symbol these days. You just assume that everyone understands.
In ancient Chinese thought the whole of existence is manifested through a repeating pattern of dynamic opposites. Two opposing polar forces that belong both together and separately. Wu Chi, the state before the division into these polar forces of Yin and Yang can be likened to whatever was before the Big Bang described by modern science. Yin is the name for relatively negative forces, Yang the relatively positive ones. Common examples of Yang energies are things like up, the sun, light, mountains, fast running water, a tall tree. By contrast common examples of Yin energies are things like down, the moon, dark, valleys, deep still water, a blade of bendy grass.
One can even see that Yin and Yang exists at a molecular and sub-molecular level. Any compound (e.g. water, carbon dioxide or oxygen) is a bonding of elements that hold because of a harmony of positive and negative forces (Yang and Yin energies) that emerge from a previous imbalance. Even within a single element, the atoms and their circling electrons bond together to create that piece of physical reality. So in Chinese thought reality exists because of a separation of Yin and Yang which on many different levels seeks harmony with a compatible opposite. However they can never achieve true harmony because they are all acting within the envelope of the Universe that is holding Yin and Yang apart. This original separation is the energy that fuels all that we are and all that we experience.
We are all aware of the most common depiction of Yin and Yang – the circle divided into two swirls. One is black and one is white, almost like two fishes swimming in circles. In addition the white (Yang) contains a spot of black and the black (Yin) contains a spot of white. This is a very clever representation of a multi-dimensional concept. The inventors of this symbol are trying to convey, not only the separation of Yin and Yang as discussed above, but also the constant interplay between the two forces that occurs when you add the dimension of time. If you start at the tail of Yin for example, you find that over time a force can become more and more yin. But when it is most Yin (at its fat head) you become aware of a spot of Yang and it you push further into Yin – bam! - it convert to being a Yang force. In my interpretation of this diagram I like to imagine that the spot is the tail of the next energy appearing like a whirlpool or black hole, that appears as you approach the heart of the current energy.
To reiterate, although we can see there is a separation of Yin and Yang, in essence there is also no separation, just interplay between two forces that must join and become one, yet cannot due to the greater force of, let us say, the Big Bang.
An understanding of these forces underpins all of ancient China's knowledge: from the philosophy of Taoism; to art, medicine, engineering and all manner of martial arts and strategy.
Within Tai Chi most people's work (whether they know it or not) goes into understanding two major Yin/Yang systems:
The change and interplay of Yin and Yang energies in your own mind/body system
The change and interplay of Yin and Yang across two merged mind/body systems (the opportunity to merge arises when you have contact with an opponent).
In order to explain these I want to start by talking about how Tai Chi demands a physical understanding of Yin and Yang. Our bodies must stand in a state of balanced Yin and Yang so that we are always stable. We never throw our weight as is often done in the external martial approach, because from the point of throwing to the point of recovery we have no rooting, no stability and therefore cannot change. As a result a good practitioner can attack at any time and change their mind to do something else at any time. Once you have this ability you won't want to do anything that compromises it. In Yang style Tai Chi this ability to always balance Yin and Yang as we move about is learnt through slow large circle form practice. The constant weight shifting from one leg to the other combined with different types of waist turn and arm and leg movement gives you practical knowledge of how to stay centred from extremely compressed (overall Yin) to extremely stretched (overall Yang) postures. Small circle forms assume you have already experienced the extremes of Yin and Yang movement elsewhere.
Tai Chi movements physically balance Yin and Yang to create stability. Within that balance how stretched or compressed the movement is from our normal standing position is one of the factors that dictates whether the overall expression of the posture is labelled Yin or Yang. For example a common posture like an extended Ward Off is Yang, whereas something like the drawing, compressing action of Rollback is Yin.
From a stable posture the soft usage of muscle and intention focussed from the centre outward beyond the physical limits of the body leads to stronger and stronger energy flow and balanced projection of energy ('Chi'). Our Standing Post Chi Kung meditation teaches us how to project energy through intention from a fairly neutral posture. When we make the posture many times more Yin or Yang (e.g. an extreme Ward Off or Rollback) there is a many fold increase in the power of the projected energy.