The chapter is four sentences long and its argument rests on a single observation about penetration. The softest thing in the world rides roughshod over the hardest, and what has no substance enters where there is no gap.
The second clause is the sharper of the two and the more literal. Water gets into stone, and it does so not by being stronger but by having no fixed shape to be stopped. Air and light do the same. The claim is not that soft defeats hard in a contest, which would still be a contest; it is that something without substance is not subject to the barrier at all, because a barrier works by contact and there is nothing for it to contact.
From that the chapter draws its conclusion in one line: by this I know the benefit of doing nothing. The inference is compressed and worth spelling out. A hard object trying to get through a wall must overcome it. Something insubstantial arrives on the other side without an act of overcoming having occurred anywhere. Non-action is being presented as the mode of operation that has no point of resistance to generate.
The chapter closes by naming the two things it has been describing and admitting how rare they are: teaching without words and the benefit of doing nothing are matched by few in the world. Both phrases are picked up from chapter two, and putting them together here identifies them as the same practice in two settings, one instructional and one general.