Okay, it's NOT nitrous. It's just a bottle that stores fuel (normal mix if it's 2-stroke) and dumps it in when the engine demands it. So, let's say you're running lean in the top end. Well, if you're really winding it up, it lets some of the fuel in to even out the mix and give you that little extra bit of power you need where you need it.
At least, that's my understanding of it. Nitrous is different from a boost bottle.