3 The Movement of the Dao

The motion of the Dao is reckoned as having a dialectical character that reflects the growth, change and decline of all things in a developmental cycle. The idea associated with "reversion" (fan), if not absolutized as it is by Lao Zi, can still have a valid message even judged from a modern perspective. Here we focus on Chapter 40 (DDJ).
3.1 (Chapter 40)
Reversion is the movement of the Dao.[1]
Weakness is the function of the Dao.[2]
All things under Heaven come from Being-within-form.[3]
And Being-within-form comes from Being-without-form.[4]

Another important principle from which has derived major fundamental principles in Tai Chi chuan: particularly in Sun Style Tai Chi Chuan, the concept of "Opening/closing" is in perfect correlation with developmental cycle described above.
From this principle, the perfect performance a taolu of Chen Style Tai Chi should reveal this "pulsing and round" impression of power, instead any "continuous and linear" issuing of energy specific to any "external martial art".

Annotations:
[1] The concept of fan (reversion) contains a similar meaning in this context as it does in Chapter 25 (da yue shi, shi yue yuan, yuan yue fan--The great is boundless and thus functioning everywhere. It is functioning everywhere and thus becoming far-reaching. It is becoming far-reaching and thus returning to the original point). In addition, it is used here to signify a dynamic and circular movement of the Dao according to such contemporary Lao Zi scholars as Gao Heng and Chen Guying.
[2] The term ruo (weakness) is ambiguous as well. There are so far a number of interpretations of this, of which we cite three key ones as follows: firstly, it is supposed to denote the function of the Dao that exemplifies itself through the soft and weak; secondly, the function of the Dao lies in helping all things grow and become complete naturally without any imposing force or pressure; and finally, the function of the Dao displays itself by the dialectical fact that the soft and weak are to overcome the hard and strong as is expressed by Lao Zi in rou rou sheng gang qiang. 1 am inclined to agree with the third interpretation since it corresponds to Lao Zi's philosophy of shou rou (keep to the soft and weak) as a whole.
[3] The Chinese character you is rendered here as Being-within-form identical in meaning with the same concept figuratively described as "the mother of all things" (wan wu zhi mu) in Chapter 1 (DDJ).
[4] The notion of wu (Being-without-form) means the same as it does in Chapter 1. It implies the original state of the Dao, which is characterized by invisibility and yet with initiative potentiality.

Commentary:
Brief and concise as it is, this chapter is strikingly rich in connotations. It is generally concluded that Lao Zi exposes his dialectical concept of the Dao in terms of its movement and function, which in turn comes to be a law of change and transformation from one side to its opposite.
According to observations by Che Zai, Chen Guying and others, "reversion" (fan) refers to a kind of interrelation between opposites in one sense, and in another sense, a kind of return to the root known as the unity of opposites. The former conveys the meaning of being opposite while the latter, the meaning of transformation and change. We think that the movement of the Dao in such a manner of "reversion," may be well symbolized by the tai ji, in which the two forces known as Yin and Yang are always in motion, inter-depending and interacting at the same time. The generalization that "reversion is the movement of the Dao" can be seen as a refined version of what is said about the Dao in Chapter 25 (see Annotations [1] above). It is noticeable throughout human history that things (i.e. a nation, culture, economic strength, political power etc.) are doomed to roll downhill once they reach their acme. This indicates that they tend to reverse to their opposites in an ever-changing process. I￡ we give due consideration to our surroundings, for example, the changes detected in plants and the stages experienced in the life cycle, we may collect sufficient evidence to justify the dialectical movement of the Dao. It is schematically interesting to quote a well-known Chinese saying from The Book of the Hou Han Period (Hou Han Shu: Huang Qiong Zhuan): "Things that are too high fall down easily; things that are too white stain easily; songs that are too pretentious have few listeners; reputations that are too lofty often fall short of reality" (Cf. Chinese Maxims, p. 129). All these possibilities seem to be in conformity with the Chinese conception of "Inevitable reversal of the extreme" (wu ji hi fan).
It is worth pointing out that Lao Zi, even though emphasizing the opposing interrelationship between things and the significant role of their transaction or transformation, ultimately focuses on the idea of returning to the root as the final destination for all things. For it is right there in his ideal that absolute stillness. tranquility or state of take-no-action will be realized and actualized, and accordingly all the conflicts and antitheses in the world will draw to an end.
The statement, "Weakness is the function of the Dao" is in fact a further justification of the foregoing assertion that "Reversion is the movement of the Dao." Lao Zi's philosophy features a preference for "cleaving to the soft and weak" (shou ruo). He often uses shui (water) as an image when illustrating the overwhelming power of "the soft and weak." It is helpful for gaining a better understanding of this notion if we approach it with reference to his discussion in Chapter 78 (tian xia zhi ruo mo guo yu shui, er gong jian qiang zhe mo zhi neng xian....-- Nothing under Heaven is softer and weaker than water, and yet nothing can compare with it in attacking the hard and strong....).
However, one should be aware of the problematic aspect of Lao Zi's confirmation that the soft and weak is bound to conquer the hard and strong. This is largely due to his absolutization of the former by cutting it off from actual and varying circumstances or conditions in both subjective and objective dimensions.
Also offered in this chapter is a generalized explication of how all things "under-the-sky" come into being. It is here once again that Lao Zi traces the origin of the universe. Both Being-within-form and Being-without-form are different names for the Dao, and are likened to the two sides of the same coin. In short, the expression in this context is a modified as well as a condensed one of the ideas presented in Chapter 1 (see Part 1, l.1).