According to Lao-Tzu, the Tao is what gives rise to the Ten Thousand Things (wan-wu); it penetrates all of existence; it "acts without acting" (wu-wei). How, though, can anything act without acting? The best analogy I have seen to explain this is one from Prof. Robert Henricks, who compares the Tao to an uncultivated field, and the Ten Thousand Things to wildflowers. The field does nothing, it does not act... but the wildflowers can not exist without the field. So it is with the Ten Thousand Things and the Tao. The Ma-wang-tui slips refer in Chapter 6 to the "valley spirit" - this is likely another name for the Tao. Here is Chapter 6 in its entirety:

The valley spirit never dies;
We call it the mysterious female.
The gates of the mysterious female -
These we call the roots of Heaven and Earth.
Subtle yet everlasting! It seems to exist.
In being used, it is not exhausted.

Like the field under the wildflowers, the valley spirit is used by the Ten Thousand Things, but they are not necessarily aware of it. All one can say of the Tao is that "it seems to exist" - there is no physical evidence for its existence. It is still, empty, and inexhaustible. The flowers are nourished by the field, but do not exhaust it. Likewise, the Tao enriches our lives, but there is no way we could ever "use up" the Tao.