I saw that verse 42 is discussed in the Voices of Experience section, where I have no clearance to post - so I do it here, instead
It must be noted that the verse commences with a cosmogony, to which the yin and yang part belongs. Roughly translated, it reads:
Tao begat one, one begat two, two begat three, three begat the ten thousand things.
The ten thousand things carry yin and embrace yang. They reach harmony by uniting in the life spirit (qi).
The rest of the verse talks about other things, and was probably once a separate verse.
Although the lines above should be read as sort of a creation story, they obviously fit aikido quite well. Uniting in the life spirit - that's pretty much what we do. Also the pair of yin and yang can be compared to that of tori and uke, needing to find balance together, and being quite incomplete without one another.
Indeed, a good verse to contemplate in one's aikido.