One of the problems of Taiji, especially Yang style Taiji, is collapse in the upper spine. This will tend to happen when the practitioner forgets that spirit should be the driving force for practice, and not habit or duty. Spirit – life force – is the energy that prevents Yin and Yang fusing into a grey and bland monoculture. Without spirit Internal and External will become mere reflections of each other instead of the complimentary opposites they are meant to be. The Taiji Classics are adamant on this: Outside soft like a beautiful woman, inside like a tiger ready to pounce. The outside always disguises the inside. So the form of Taiji is slow and soft and relaxed and sunk but the inside is quick and hard and tense and rising. What you see is not what you get. Nature loves to hide. Trickery and subtlety are built in to the natural process.
In our struggle to maintain a mental grip on the world we try our utmost to keep the head still. Then the world is simply what we sense rather than what we are a part of. Hardly surprising then that the ego – the capitalist inside us – sees the world as a source of profit rather than as something to belong to. One way out of this is to relax the neck and let the head and the senses swim, as though ever adrift.
Taijiquan was developed as a martial art many centuries ago. In all likelihood it was practised by vigorous young men who worked as farmers, that is, by men already physically strong with good fiery spirits. These men would have been taught relaxation and turning so that their strong bodies could borrow and return energy. Hours and hours of pushing hands would have taught them to listen and stick to their opponent, and hence yield/attack was born (can't have one without the other). Nowadays, Taiji is practised by middle class office workers: well educated, well meaning folk with weak slack bodies and slumbering spirits, full of neurotic tension and overactive minds. Such students require a different approach to those of old: focused meditation to quieten the mind, vigorous labor to strengthen the body, and some form of fighting to tone and hone the spirit. Otherwise Taiji has no authenticity, no heart, and the wrong sort of mind.