[TooFastTim]

Power valve covers: the power valve cavity acts as a resonant cavity (as does the expansion chamber), the bigger the volume the lower the frequency (RPMs) for which the engine will be tuned. The pipe is also tuned for a particular frequency. That's why pipes on 250's are longer than those on 125's. They're tuned for lower rpms because 250's are lower revving than 125's. The power valve effectively adjusts the volume of the power valve cavity. So the entire system has one variable tuned cavity (power valve) and one fixed tuned cavity (pipe). If you're an electrical engineer it's like having two tuned circuits. Whether in parallel (which I suspect) or series I don't know.

I've always worked on the principle that the bike is almost certainly better than I am so I adjust my riding technique to suit the bike.