To understand the purpose of this discussion, it is worth remembering a few simple concepts of Taoist metaphysics.
Yang, volatile and intangible, can only remain stable in a manifestation (like a human) if the yin and structural materials is proportionally present: basically, I can only put in a bucket (yin) the volume of liquid (yang) that fills this space. A low or limited yin only permits a little stable yang: the body (yin) should be worked in order to hold the energy (yang).
The structural (yin) must be leak free for the yang (function) to be at its maximum potential. The physical (yin) holds the spirit (yang); the essence (yin) is the root of the mind (yang).
It is said in a text of the fourth century that the internal force is not trained, it only depends on the work of the external form: energy is abundant when the form is just.
We give a completely fair importance between the three facets of our human incarnation: the physical, the breath and the spirit (jing, qi and shen): it is a real specificity of the Taoist arts and Chinese thought.
To develop strength of mind, it is necessary to have eliminated the limitations and bottlenecks of the physical form, otherwise these will be the reasons for the stagnation in our practice. The ability to develop and maintain the energy (the breath) generated during the exercises depends directly on the state of strength, rootedness and relaxation of our body ... The yin is the root of the yang.
To ensure this, we are advised to work the form to such a level that the substance is there: the qualities of the physical form will allow the qualities of the energetic function.
What this means in simple terms?
My body shape should be conducted towards perfection and this search guarantees me abundant energy. My physical movements must be trained in a continuous search for strength, rootedness and relaxation to accommodate the inner strength, the strength of qi. My movements and forms must be beautiful and must give an impression of strength in my actions!
That is why it is good to work complex and detailed forms: it is easier to fix and see its flaws. When I work a form too stripped or even static positions, it is very difficult to see what is wrong: these forms are ideal when the level of practice is already high. And in general, those who have more work to do on their body form are those who choose a simplified form: they can thus ignore their flaws.
The work of the forms of Taijiquan or Baguazhang are ideal for seeing and correcting our imperfections: it suffices to look to see them. The work of the old forms (daxuan) or of the static positions do not allow this: it is easier not to make any progress and stay in a comfortable mediocrity.
However, after a mastery of the physical qualities, the work of complex forms is an obstacle to energy work. To continually evolve and be able to correct oneself easily and see our flaws every day, it is important to keep a work of the complex forms: the constant improvement of one's body shape allows a continuous and stable evolution.
The sublimated Yin will enable to "keep" the yang: the daily exercises will be a great source of vitality that will be kept in the trained body.
In order to progress, to correct one's yin, it is important to see one's faults and the complex forms (Taijiquan, Baguazhang, water boxing...) allow this.
It is also possible to hide behind advanced forms and thus go nowhere.
In the perfection of the dominated external lies the root of true internal practice.