what does the allusion "knees that launched a thousand ships" refer to?

Here is the background of the original phrase:
http://www.phrases.org.uk/meanings/359800.html

What you quoted is alluding to this in some way.

Helen of Troy, the most beautiful woman in history, was "stolen" from her husband, and the country went to war to go get her back. A navy of a thousand ships sailed to get her.

http://library.thinkquest.org/04oct/00018/Cosmic%20Odyssey/TMorikawa/IliadSummary.html

But that was a face that ...
Here we have knees that ...

I don't remember how that fits into this book, it has been so long since I read it.