Powervalve controls when the "powerband" kicks in, be it immediately upon rolling the throttle on (1/3 throttle turn) or later (2/3 throttle turn). It's that point at which the back wheel suddenly starts spinning, the motor sounds like it's breathing fire, your elbows pop and your head snaps back.....oh wait, you're riding a 125! ;)
This is of course speculation, be it that I have no rm125.
It is an adjustment for the tension for the powervalves. As you turn it clockwise it increases the tension and delays when the PV opens. Set at the stock setting and leave it there. Get the manual and it will say what is stock.