Once you installed windows 7 on your computer you will notice that there is nothing special in it. That's because all the cool features are hidden. So you will need to download this tool and run it. Then restart and you will see the hidden features like taskbar icons and quick preview etc are revealed.
It really is a cool program that fakes windows 7 to thing that its being run by an authorized Microsoft employee and that is how these features which will be released with windows 7 are revealed.