The valves are controlled by engine vacuum and throttle opening. When the carburetor slides are raised less than 3.3mm, a valve located at the carburetor linkage remains closed. With that control valve closed, vacuum from the intake manifolds opens a dashpot located underneath the carburetors, behind the cylinders. That dashpot pulls butterfly valves closed via an adjustable rod linkage which runs underneath and between the cylinders to the exhaust valve assembly.
When the carburetor slides are raised more than 3.3min, the control valve opens a line leading from the airbox, dashpot vacuum drops, and the butterfly valves spring open. The valves are either open (under most conditions) or closed (under deceleration and very small throttle openings) and do not hold at any position m between. When the control valve is open, the line from the airbox constitutes a coitrolled air leak, which is compensated for in carburetor jetting. (A one-way valve in the intake manifold vacuum lines prevents any reversal in the emissions system air flow.)
The valves are effective because a twostroke is less efficient with the throttle closed, especially when decelerating. The rich air/fuel mixture tends to run right through the cylinders and out the exhaust without complete combustion, and the lack of compression braking effect allows a decelerating two-stroke to coast with the throttle shut for relatively long distances. That adds up to lots of emissions during deceleration. The butterfly valves work on a sort of potato-up-the-exhaust-pipe principle - with the exhaust plugged, emissions aren't a problem. In actual fact, it isn't that dramatic. When open, the exhaust manifolds are 1-7/16 in. diameter, for a combined exhaust area of 3.25 sq. in., not counting the obstruction caused by the open butterfly valves. With the valves closed, exhaust exits through two quarter-inch holes in each valve, for a combined exhaust area of 0.2 sq. in. It isn't a case of total blockage, but it gets the job done.