The valley spirit dies not, aye the same;
The female mystery thus do we name.
Its gate, from which at first they issued forth,
Is called the root from which grew heaven and earth.
Long and unbroken does its power remain,
Used gently, and without the touch of pain.
成象, 'The Completion of Material Forms.' This title rightly expresses the import of this enigmatical chapter; but there is a foundation laid in it for the development of later Daoism, which occupies itself with the prolongation of life by the management of the breath (氣) or vital force.
'The valley' is used metaphorically as a symbol of 'emptyness' or 'vacancy;' and 'the spirit of the valley' is the something invisible, yet almost personal, belonging to the Dao, which constitutes the De (德) in the name of our Jing. 'The spirit of the valley' has come to be a name for the activity of the Dao in all the realm of its operation. 'The female mystery' is the Dao with a name of chapter 1, which is 'the Mother of all things.' All living beings have a father and mother. The processes of generation and production can hardly be imaged by us but by a recognition of this fact; and so Laozi thought of the existing realm of nature—of life—as coming through evolution (not a creation) from the primal air or breath, dividing into two, and thence appearing in the forms of things, material and immaterial. The chapter is found in Liezi (I, 1 b quoted by him from a book of Huangdi; and here Laozi has appropriated it, and made it his own. See the Introduction, p. 2.
From Qigong Meditation: Embryonic Breathing1
The Valley Spirit (Gu Shen) does not die, then it is called "Xuan Pin." The door (i.e. key) to reaching this "Xuan Pin" is the root of heaven and earth (i.e. nature). It is very soft and continuous as though it were existing. When used, it will not be exhausted.2