The power valve is all about low end torque and high end power. Basically when the valve is fully open it allows more exhaust gas to flow and thus more power in the high RPM range. However it remains mostly closed during low rpm restricting exhaust gas flow and creating more low end torque.
The point i'm trying to make is that the valve opening should be gradual, and if it's opening a little during low-mid rpm and then all the way during full throttle, then it's functioning correctly.
Usually a power valve malfunction is when it's stuck in a particular position, usually fully closed or fully open cause it's really dirty. If it's fully open then your gonna have **** all for low end torque, and if it's stuck closed then your going to have the proper bottom end torque but no high end power.
Hopefully you understand how it works in which you can correctly diagnose if it's working properly.
Hope that helps.