The way is empty, yet use will not drain it.
The translator has substituted drain for full because this is in keeping with three other lines about the way not being drained. These other lines are in different contexts which are not to do with the emptiness of the way. It is perhaps possible to make something of the translation but let's revert to the original.
1. The way is empty, yet use will not fill it.
Why should using something fill it? Feng and English have an interesting translation for this line:
The Tao is an empty vessel; it is used but never filled.
In order to use a vessel it needs to be filled but not so with the Tao.
The way is empty. What is it empty of? It is empty of notions and concepts and all things are notions and concepts so the way is empty of all things. The way has nothing to do with notions and concepts.
How can the way be used? The way can only be used by following it, by practising it. Practising the way is not done in a vacuum. It is a way of living life and in life we have to deal with things. These things might seem in contrast to the way but the practice is to take them on board and to see them as of the same nature of the way, that is, as empty, as devoid of any intrinsic, independent reality. It's possible for the practitioner to become 'filled up', to become identified with notions and concepts, and thus to deviate from the way but if these lumps of attraction can be dissolved or at least temporarily bypassed then the way is clear and the way is always empty.
2. Deep, it is like the ancestor of the myriad creatures
Deep - as in 'fundamental'. Everything stems from it so why is it only like the ancestor of the myriad creatures? The myriad creatures propagate each other and theoretically they could have a single ancestor. But this single ancestor is not the way. The way is only like the single ancestor because the myriad creatures depend upon it.
The purpose of this line is to direct us to the correct attitude for moment to moment practice. The relationship of the way to the myriad things is like the relationship of the sun to the things of this earth. The sun is completely unaffected and indifferent to the things of the earth yet we are critically dependent on the sun. The relationship between the way and the myriad things is even more intimate though because this dependence is only illusory inasmuch as the myriad things are essentially non-different from the way. If this is not perceived then the practitioner could get diverted into a dualistic egoistic struggle.
These first two lines complement each other. In the first line all things are 'dissolving' into the emptiness of the way. In the second line all things are 'propagating forth' , fundamentally dependent on the way.
Our problem is that we lose the way amongst the myriad creatures. The first line exhorts us to keep coming back to it, the second indicates that by not endowing the myriad creatures with lives of their own and, instead, recognising their fundamental nature, we can come back to the way. More specifically...
3. Blunt the sharpness;
This is exactly what needs to be done. Our attention is like a sharp needle directed towards things be they material objects or mental objects (thoughts, ideas, fantasies, etc). Sharpness implies distance:
Bluntness collapses the distance of the pointedness,
bringing us right back to Here.
Untangle the knots;
When not Here, our consciousness is there, lost, identified with phenomena, tangled up, stuck, knotted in phenomena. Sometimes it is difficult to come back to the Here and Now. Sometimes we're entangled in a knot that actually believes it's the Here and Now. Constant renewal of effort, constant exercise of awareness, is required.
Soften the glare;
This is like the sharpness - the glare of the there takes us away from Here. Like jewellery, multi-faceted phenomena fascinate and hypnotise us. The glare of phenomena blinds us to the Here and Now.
Let your wheels move only along old ruts.
With old ruts you don't need to worry about steering or getting to a particular destination. No need to be concerned about the direction. In fact, give up any notion of steering, of finding a direction. The ruts are old also in the sense of 'fundamental'. It's ante-cognitive. It's the way of no neurotic effort. It's actually an effort to move away from the way. The natural state is to be Here, Now. Our efforts are habitual but when these are brought to awareness and given up then things take care of themselves: the old ruts carry us along.
4. Darkly visible, it only seems as if it were there.
Darkly visible - like a sense of something in the dark, like an intuition of some thing. We can begin to talk about it and objectify it, perhaps as a presence of some sort. It's no such thing of course. It isn't even an 'it'.
5. I know not whose son it is.
Even so, in talking about it, as a presence perhaps, it is here described as a son. Rather than stating that this son has no forefathers the text implies that these forefathers are not to be known. This son is the bottom line.
6. It images the forefather of God.
The word God doesn't strike me as particularly helpful here. What is it referring to? The prime ancestor perhaps, the progenitor of the myriad things? If so, this hearkens back to the second line. If God is the ancestor of the myriad creatures, the creative principle in fact, then the forefather of God is what comes before the creative principle, that is, before the cognitive process.
Images - to make an image of. It images this ante-cognitive perspective. The way is showing itself, making its presence felt, albeit in a darkly visible manner.
Bearing in mind that the Tao Te Ching isn't about speculative metaphysics, these last three lines are reminding us and directing us to the heart of experience where we've been before. The sense of Self, of Being, of Presence is the darkly visible image through which we arrive back Here.