To what extent can this statement refer to cultivation?
This would be mere speculation, but through my own experience, there is a reversal and transformation of the three essentials in the body, which would lead to a liberation of the self, as conscious death. But to what does the valley spirit refer?
It is said that when you can cross the line of life and death at will, you will see that they are one and the same, and thus you become and immortal. But your physical body still dies when the Jing is used up.
Does the text reveal a way to replenish the prenatal Jing, or is it pure metaphor?
The way i understand it, valley spirit is yin spirit (vally-yin, mountain-yang). From what I’ve read ghosts are usually composed of yin spirit – some managed to store a bit of yang and so they often ‘appear’ to untrained people (this is quite rare) – however to see a fully yin spirit (and allow other untrained ppl to see it) you must have a LOT of yang so that the ghost can ‘borrow’ some.