A powervalve is a mechanical device used to increase or decrease the size of the exhaust ports on a 2 stroke engine. Originally 2 strokes had no powervalves (not to be confused with powerband) and the motors had no real flexability. These motors either had to be designed with low/mid range power or all midrange power or Mid/top power. The manufacturers descoverd that if they could vary the size of the exhaust port they could have a more flexable motor. The valves are driven off the RPM's, at low RPM the valves are lower which makes the exhaust port small for better low RPM throttle responce. As you increase RPM's the vlaves gradually open increasing the exhaust port producing better midrange power.Once full open the ehaust port is flowing maximum exhaust gases and producing peak power.
If you look at a current MXer you will see just above the exhaust header (where it attaches to the cylinder ) a section that houses the Powervlaves.
They do require maintenance to clean off the oil residue and carbon buildup if their is any.
I had a 2000 RM250 which is the same as yours and the bike was really great. Never had any problems with it and it was very fast. This past spring I sold the bike and purchased an 02 RM250 from a local guy that wanted a 4 stroke (yuk). I Love riding MX and hope I can get the old form back and do some District 6 races in the near future. Enjoy the bike and keep a maintenace log book with the dates and what you fixed/replaced or tuned on the bike. It makes it easier to keep it running good that way.