This seems at the first sight to be somewhat mystic. However, with regard to the non-observable aspect of the Dao, this shows that the Dao is omnipotential.

The significance of the expression "the Dao of old" lies in its efficacy in "harnessing present things." This indicates that the Dao in the abstract stays potentially in power for ever. When followed by man, it helps him to tackle social issues, and govern a state or even the whole world in the most effective and peaceful manner. That is why Lao Zi strongly recommends people to "hold on to it." It is worth mentioning that such characteristics of the Dao (or the inseparable One) described as "the Vague and Elusive" (hu huang) happen to be rather perplexing to many readers of Lao Zi because they are so nebulous. Nevertheless, those who have a sound knowledge of Chinese qigong would be most likely to understand the wording employed by Lao Zi.
Qigong, Tai Chi Chuan is known as a system of mental and physical exercises involving breathing, with the purpose of spiritual nourishment and personal cultivation, and even self defense for Tai Chi Chuan case.
It has been practiced in China since ancient times. Similar to meditation, it attempts to purify the mind and free it from inner tensions and desires, and even external temptations. Lao Zi advocates getting rid of selfishness and desires in order to return to the primeval state of simplicity and tranquility. "The Dao of old" can also viewed as older people will rather practice it that younger generation.
But those who has the chance to adhere to tai chi real principles and practice it since their childhood, will always reveal uncommon flexibility, agility, general adaptability but also fighting aptitudes.

2.2 (Chapter 35)
If you hold fast to the great image,[1] All the people under Heaven will come to you.[ 2]
They will come and do no harm to each other, But will all enjoy comfort, peace and health.
Music and dainties can make passers-by tarry,[3]
While the Dao, if spoken out, is insipid and tasteless.
Being looked at, it is imperceptible.
Being listened to, it is inaudible.
Being utilized, it is inexhaustible.

Annotations:
[1] The term da xiang (great image) refers to "the image without image," according to Lin Xiyi's annotation. It is in fact used for the Dao or the manifestation of the Dao which functions as the guiding principle for a successful ruler in a social sense.
[2] The expression that "All the people ... will come to you" is meant to demonstrate people's willingness to become citizens of a state under the leadership of a ruler who has embraced the Dao.
[3] "Music and dainties" (yue yu er) are apparently as appealing to the senses and desires of the people as the notions of "humanity and the rites" advocated by Confucianism in Lao Zi's time.

Commentary:
This chapter intends to exemplify the engaging power of the Dao in a social sense. A wise ruler who grasps the Dao is likely to win the hearts of all the people under Heaven; he will be then in a position to bring peace and order to the whole world, and accordingly make people feel secure, protected and happy. Unlike tangible and attractive things, for instance, "music and dainties" or "humanity and the rites" as fine-sounding promises, the Dao is insipid and tasteless, imperceptible and inaudible as well as inexhaustible. Nevertheless, it is capable of making the world peaceful and its people content.

Real efficiency in internal martial arts, looks invisible from any outsider, even during the taolu practice, how can these slow and graceful movements may defeat strong opponent?
Unfortunately we cannot find words to describe the progressive benefits to one's health but also to one's real aptitudes in combat, it is only through sincere practice that this reality will become yours...

2.3 (Chapter 21)
The character of the great De[1] Follows from the Dao alone. What is called the Dao Appears elusive and vague.[3]
Vague and elusive as it is, There is the image in it.
Elusive and vague as it is, There is the real in it.
Profound and obscure as it is, There is the essence in it.[4]
The essence is very concrete And contains the proof inside itself.[5] From the present back to the past[6]
Its name continues to ever last, By which alone we may know the beginning of all things.[7]
How do I know then beginning as such?
Only through this.[8]

Annotations:
[1] The original term kong de is translated into "the great De" and is regarded as the manifestation of the Dao. Its quality is all-embracing, operating in everything and everywhere.
[2] De works in conformity with the Dao simply because the former is the manifestation of the latter.

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