It works by converting your RSS URL to an alternative, so it works regardless of what RSS reader you use. Though it is only worth using on feeds that only give partial articles or just links. To use it, take the feed's URL and put it in the box, then press the button. The site will convert the URL into a new one (it will be in your address bar as it both produces it and navigates to it). Take this new URL and put it in your feed reader. It will now download the full text for each entry instead of just a preview. This does not work on all feeds though.

Example, this forum has a feed:
http://www.entourageedge.com/forums/....php?type=RSS2

It converts to:
http://www.wizardrss.com/feed/www.en....php?type=RSS2