The second half of the chapter turns from description to warning, and the warning is aimed at people who have taken the description as a target.
Knowing harmony is the constant and knowing the constant is clarity. Then: to add to life is called ill omened, and to force the breath with the mind is called violence. And the standing formula, that a thing which grows strong then grows old.
Two of those phrases became technical terms in traditions that came long after. Adding to life names the attempt to increase vitality, and forcing the breath with the mind names the deliberate direction of the breath by intention. Both are recognisable descriptions of practices that later Chinese cultivation traditions developed in great detail, and this chapter condemns both in the strongest words available to it, calling one ill omened and the other violent.
That is a genuine tension in the reception of the book. The Daoist religion and the later internal arts drew on this text extensively and read chapters like fifty-two and ten as instructions for breath work, and this chapter appears to rule out the forcing of breath in particular. Practitioners were aware of the passage and read it as a warning against coercive technique rather than against practice as such, which is a coherent reading and is a reading.
What the chapter itself asserts is narrower. The infant's capacities came from a condition it was in, not from an effort it made, and any attempt to reproduce them by effort is the addition of strength to something whose whole advantage was softness.