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.