Essentially, especially for those that take up Tai Chi to improve their health or well-being, Tai Chi is on offer as an ancient form of Traditional Chinese Medicine, and as such, it is founded upon the principles and theories of Yin and Yang.
'Yin' is 'down and in' and 'Yang' is 'up and out'. Yin = defensive, Yang = offensive. Yin = soft, Yang = hard. An in breath is a Yin breath and an out breath is a Yang breath.

Our limbs are said (in TCM) to be Yang on the outside and Yin on the inside.
pic 1: The unbroken lines represent Yang and the broken lines indicate Yin.
pic 2: Attention. We stand with our feet together and with palms flat against the thighs.
The longest finger on each hand points down the seams of your trousers. For me, this posture personifies "stiff, straight world".
pic 3: Preparation. Next, by assuming the posture of Preparation, we step into the world of Tai Chi - which is "smooth and curved".
We transfer our weight into/over our right leg and, in a smooth and curved motion we lift our left leg, place it back down about a shoulder width from the other and make sure that we then transfer our weight to the centre point between our legs and 'sink' by bending our knees a little. As we go from Attention to Preparation we turn our palms to be facing backwards, ending with our thumbs resting gently against our thighs.

In the process we 'unlock' our Chi and allow it to flow through our limbs and torso, yin on the inside and yang on the outside - and, with our palms and a greater proportion of our forearms now facing behind us, and with the aid of Circular Breathing techniques and visualisations [session 2], we engage in the Opening of the Grand Terminus; picturing the Yin or in breath behind us and the Yang or out breath in front (pic 4).

As we go from the stiff, straight world of Attention to the smooth curved world of Preparation and complete the "Opening of the Grand Terminus"; and we imagine that we have just stepped into water, causing ripples on the surface and disturbing the sandy bed.

At this point it is suggested that, to become in accord with Tao or 'the way of least resistance', we should remain stood there until we can once again see our own reflection in the water ... and begin to feel your feet sinking into the settling bed. The latter is a metaphor for the sinking of Chi into our Belly, Hara or lower Dan Tien, and the former for the exquisite self examination that Tai Chi promotes and encourages.