Well, I can't be bothered to read this but the way I *assume* it works is to rewrite the page to add the new link into it for "unfold" and then when that link is clicked it does a http requesdt to the server for the page, strips out the comments it needs and puts them in the page with magic javascript. The sensible way of doing it will only produce more server load because it is easier to read the comments and so people will do so more often.

That does of course assume they are doing it sensibly. If they're not then they are crazy. And I can't really think of any practical way to do it non-sensibly anyway. :)