tao teh ching 55

He who is filled with Virtue is like a newborn child.
Wasps and serpents will not sting him;
Wild beasts will not pounce on him;
He will not be attacked by birds of prey.
His bones are soft, his muscles weak,
But his grip is firm.
He has not experienced the union of man and woman, but is whole.
His manhood is strong[1].
He screams all day without becoming hoarse.
This is perfect harmony.

Knowing harmony is constancy.
Knowing constancy is enlightenment.

It is not wise to rush about.
Controlling the breath causes strain[2].
If too much energy is used, exhaustion follows[3].
This is not the way of Tao.
Whatever is contrary to Tao will not last long.

[1] Borne out by the Ma wang tui reading, Lau reads this as:

It does not know of the union of male and female yet its male member will stir:
This is because its virility is at its height.

and then goes on to read the next two lines in the same way — it can yell all day and still not have a sore throat because its harmony is at its height.
[2] Both texts also agree that the four preceding lines here should be grouped:

To know harmony is called 'the constant';
To know the constant is called 'discernment' ('being wise');
To try to add to one's vitality (add on to life) is called 'ill–omened';
For the mind to egg on (to control) the breath is called 'violent'.

[3] Lau reads this as 'A creature in its prime doing harm to the old', the Ma wang tui text as 'When things reach their prime, they get old'.

Wang Pi says:

The infant is free of craving and desire and so commits no offence against the myriad things. As a result, no harmful creature will commit an offence against it. So too, since one who has fully internalised virtue commits no offence against others, they, in turn, do not seek to make him lose his wholeness.
It is because of his softness and pliancy that his grip can be so completely firm.
Because there is nothing to make him suffer bodily loss, he is able to remain completely erect. In other words, for one who has perfectly internalised virtue, no–one can cause him to lose his internal strength or dilute his authenticity. The soft and the pliable do not contend and are thus never broken. It always happens exactly like this.
His heart/mind is free of contention and thus he can sound forth all day and yet never grow hoarse.
People achieve constancy through balance. If one knows how to maintain balance, one will achieve constancy.
Constancy is neither bright nor dark, neither warm nor cold. Such a one remains formless, so what he is cannot be seen. Therefore the text says: 'To understand constancy is called perspicacity'.
Life must not be extended. One who tries to extend it will always 'meet with an early death'.
The heart/mind should consist of nothingness. If one tries to control the vital energy, this is called 'forcing strength'.

Professor Cheng comments: This chapter praises the fullness of an infant's virtue (teh): its undissipated vital juices and its unimpaired harmony. This is based on reality and is not just empty words. Whereas a real understanding of harmony will bring constancy, without harmony, constancy will eventually disappear. Since knowing constancy brings enlightenment, those who lose constancy also lose the requisite for wisdom. Just as that which is beneficial to life is of good augury, that which is harmful to life is inauspicious. Weaklings who can mobilise ch'i through the heart will inevitably become strong. Although Lao–tze generally disapproves of (the search for) the 'auspicious' and 'strong', here he maintains that the ordinary person will not become 'auspicious' and 'strong' on the basis of the infant's undissipated energy and unimpaired harmony. On the contrary, 'things mature and then decay. This is counter to the Tao. That which goes against the Tao is soon finished'. The text re–emphasises these lines, which also appear in chapter 30.

His note says: 'Its bones are pliable and sinews soft' contrasts with the line in chapter 3 where it says 'strengthen the bones'. The infant is pliable and soft because it has not yet grown. The meaning of 'strengthen the bones' corresponds to the lines 'that which benefits life' and 'to direct ch'i by the heart is called steadfastness'.. This is the proper way for the Sage to educate mankind. Mobilise the generative essence (ching) to supplement the bone marrow and the bones are strengthened. This is the method to remedy prenatal deficiency.

It is far more than that!
Understanding the interplay between ch'i ('breath'), ching ('generative energy') and chi ('subtle vital energy') — how they arise, one from the other, how they interact, what they are and what they are 'for' (if one can use such a term!) — is one of the pivots of oriental yoga. When dissolving the subtle obscurations — obscurations of subtle attachment, incipient negative emotion and supposed 'knowledge' — a study of this interplay (definitely under a teacher who has realised it for him or herself) becomes extremely important.
Here we are at the very nub of Chinese alchemy. What does one 'do' with these energies? The line of thought I follow suggests that one doesn't do anything at all.
Clues as to all this — a vast subject concerning which anything I might have to say would only be opening my mouth to change step — are to be found in the book list I offered the other day (cf. the comments to ch. 38).