Reed valves are commonly used in high-performance versions of the two-stroke engine, where they control the fuel-air mixture admitted to the cylinder. As the piston rises in the cylinder a vacuum is created in the crankcase beneath the piston. This vacuum opens the valve and admits the fuel-air mixture into the crankcase. As the piston descends, it raises the crankcase pressure causing the valve to close to retain the mixture and pressurize it for its eventual transfer through to the combustion chamber.Pitchfork311 on motorbicycling.com took an $8 chainsaw reed valve and made it compatible with an HT engine!
EDIT: Please, keep in mind that you must modify other parts of your engine(like trimming part of the piston skirt) before attempting anything like this. Supposedly, you can't just slap a reed valve on a motor and call it done.
Read more about reed valves on bike engines!