An expansion chamber is one of the very best upgrades you can add to your two stroke engine. By simply bolting a good exhaust onto the stock motor you could have an immediate power boost around 30%! 

 

The science behind an expansion chamber is illustrated nicely in the gif to the top right of this page. What it essentially does is help to maximize the efficiency of the combustion cycle in the engine by preventing some of the unburned fuel from escaping the cylinder through the exhaust port like it normally would with a standard muffler. It forces the otherwise wasted fuel back into the engine with sound waves that bounce off of the inside of the expansion chamber.

 

As you can see from the examples on this page, there are very many designs of expansion chambers for the 2-stroke engines the we use on our bikes that work to varying degrees of effect. One of the best ones in my opinion that I've had great personal experience with is the one offered by SickBikeParts.com which costs about $80 plus shipping and is pictured directly below the gif image above. Many more exhaust pipes more can be researched on the internet.