Google PageSpeed Service

From the sound of it this sounds like a sort of transparent proxy service on sterioids.

Reason I say that is given "PageSpeed Service is an online service to automatically speed up loading of your web pages. PageSpeed Service fetches content from your servers, rewrites your pages by applying web performance best practices and serves them to end users via Google's servers across the globe."

It sound like it just rewrites the pages for googles servers to offer up and does not imply that they would rewrite the content of your website. So I'm intrigued in how this actualy works, but to me sounds like a transparent proxy service and in that I wonder if this only realy works out if the user uses googles DNS's. Can anybody who has used this service clarify.