there are several types of power valves for two strokes these days, Some even use side chambers (Kawasaki KPIS i think) for additional resonance. but the cool one was actually way back in the day when Sears was sell PUCH two strokes that used the "Twingle" concept. It used two cylinders with both pistons rising and falling near the same time, and both cylinders used a single combustion chamber. this allowed the intake charge sufficient room to replace burnt fuel without loosing the new charge down the exhaust port. Those engine made consistent power over a wide rev range without the need for an expansion chamber, they also ran much cleaner.
