I actually worked on developing a muffler with a butterfly valve in it for motorcycles. Ours in electronically controlled by the ECM and changes the muffler from a three-pass (valve closed) to a single pass (valve open). We originally developed it to pass noise regulations, but once we got it working, it actually helped mid-range power and torque when the valve was closed. Our valve actually is open at low RPM, closes for the mid range, then opens again at high RPM. Think of it as two exhaust systems in one. The first quiet and closed one makes great mid range but is soft at low RPM and high RPM. The second system is louder and has a big torque dip right in the mid range. By using the open-closed-open valve strategy we were able to help power everywhere, and make the bikes quiet during the noise test that happens to be run in the mid RPM range. “Active” exhaust systems are not easy to develop by any stretch of the immigration.

 

According to some folks on a BMW forum, the valve in one of the pipes on the BMW exhaust is to prevent resonating at particular RPMs and load. It supposedly has no effect on performance.