[...] instead of calling a simple GET request to save the page, since an entire page’s contents would quickly overrun any URL-length limits in the stack, it injects a FORM with a POST action and populates a hidden value with the page contents.[Source: Daring Fireball]
The whole procedure is hideously complex, but works incredibly well.
Marco Arment explains how InstaPaper works its magic. In case you don’t know: InstaPaper has a bookmarklet that, when clicked on, uploads the current web page to the InstaPaper server. Afterwards, the page can be downloaded by one of the InstaPaper apps for offline reading. Naively, you would expect the bookmarklet to send the URL of the current page to the InstaPaper server which would then crawl the contents of the page. But that would not work with sites where you need to log in. Thus, Arment implemented the following hack.