Helps acceleration from idle to top end by neutralize the pressure difference between low and high ends below the carburetor. Much smoother accelleration (no real increase in power)
I got this from tower after I did a search for "boost bottle". From what they say, it doesn't add anymore power. Just helps acceleration. Personally I wouldn't invest in one, well not yet anyway.
http://www2.towerhobbies.com/cgi-bin...?&I=LXAX77&P=7
^this is the boost bottle I got the text from.