Quote Originally Posted by CRUZER View Post
I have yet to find a good description of EXACTLY what that flapper valve does and when it is closed or open. I believe I can hear it open just after the engine starts but can't be sure. Can anyone offer a succinct summary?
Here are the words from BMW Motorcycle Magazine issue 2/2008 when they tested the newly arrived HP2 Sport (the model that first had overhead cams and first had the flapper):

An exhaust flap in front of the muffler is actuated by an electronically controlled servo-motor via a cable. This constantly adjusts the internal diameter of the exhaust system to suit engine revs and ensures good low to mid-range performance for a linear output all the way to the rev limiter.
Think about it: the HP2 Sport.

It's not a milquetoast system.

This issue of this magazine, BTW, is from when it was a German publication, i.e. before takeover by our wonderful friend in Tucson.

The K1300S has it, too.