the rotary valve cams are on the tops of the heads and have passages (grooves) cut into the sides of them. The intake is mounted to the side of the head.The grooves, as they rotate open they connect from the intake side directly to the combustion chamber. as the cam continues to rotate. another groove opens from the combustion chamber to the exhaust port. The grooves are cut in such a way that they open and close at the proper time in the combustion cycle.

HTH

Jim

OH yea, just thought about this. check out the rcv eniges web site. here's the link , click on model air plane engines. they're using rotary valves. but since it's only one cylinder, it's a much simpler desgin.

http://www.rcvengines.com/