How does this work?

I was one the internet looking at some aftermarket parts for a CRF150 wile I came apon a Thrust nozzil. It says that you get more power by putting it on, and i have no idea how that works. Would anyone care to explaine this to me? Thanks for the help in andvance. O wait, here's the link (the picture and discription of it is the very last part at the bottem of the page):

http://www.powroll.com/P_HONDA_CRF150.htm

Thanks alot, I think I'll get one of them instead of getting a pipe system :D