Harley Davidson is developing a dynamic exhaust system design that redirects exhaust gases depending on engine RPM and throttle opening. The exhaust system has 2 paths for the exhaust with a cable operated valve directed by a control unit sensing engine speed and throttle opening.
The valve operates to control the torque curve of the engine by moving the valve in the “crossover region of (the) first and second torque characteristics.”
Basically, the valve operates automatically to maintain a higher torque than would be available if using either exhaust path by itself, switching between a more or less restrictive path. Drawings indicate applications for both a regular Harley Davidson twin exhaust and the same system within the Buell under belly exhaust. The twin exhaust system has a valve either within the muffler or in the pipe before the muffler.
Besides the performance advantages the system can also be set to control noise emissions at cruising speed.
UPDATE:The differences in the exhaust paths seem to be primarily length, meaning they are tuned for different RPM ranges, whatever the differences in restriction may be.
More drawings below: