Allows you to publish content during dev phase without having to go back and fix your content’s static URLs when you switch to the production URL. Registers [url] shortcode with your site’s base URL setting. Replaces occurances of http://mysite.com/the/link with [url]/the/link inside the content editor when you hit Publish or Update.
http://chrisbratlien.com/2010/05/16/relative-site-url-on-content-save/