Anyway the tool is the Responsive Design Bookmarklet?http://responsive.victorcoulon.fr/
You just create the bookmarklet on your bookmarks toolbar. Then when you are are on the page you want to test, click the bookmark and it will apply the bookmarklet to the site. With this tool in particular, it gives you a nice little look at how your site might look at different resolutions and when a keyboard appears over the screen (when someone is completing a form for example).?
The tool probably doesn't render the pages exactly how they would on the various devices as there are so many devices and so many browsers on those devices but it gives you a quick idea anyway.?