Two stroke, piston port engines do not require an intake or exhaust valve. It is a simple design, but allows some of the burned fuel/air mixture to contaminate the fresh fuel/air mixture. Manufacturers found that two stroke engines experienced better fuel economy, reduced emissions and better lower end torque (the main reason) if they prevented the burned exhaust from mixing with the fresh fuel/air mixture. The rotary intake valve is a simply, crank driven "door" that helps to keep the burned gases from contaminating the new fuel/air mixture.
Most 2 stroke manufacturers use reed valves these days to accomplish the same thing. Kawasaki uses reed valves on their other 2 strokes, but has retained the rotary valve intake on the 100cc mill.
Maico used a rotary valve intake on their 125cc motocrosser for a few years as well.
I was going to simply give you a link to the article on wikipedia, but it was way above my head.
