3 seconds of Google yeilds:
"A power valve is just a piece of metal that moves down and covers part of exhaust port making it smaller. Big ports mean big horsepower, but they also mean narrow power band. By making the port smaller, the power valve helps make the power band wider. It does this by keeping more of the fuel mixture in the cylinder, and out of the exhaust pipe, at lower RPM."
"The power valve should be closed at Low RPM and Open at High RPM. The Power Valve does this with centrifugal weights (Governor) that over come spring pressure and move the Power Valve Linkage. The linkages usually are spring loaded. This is to close them at low RPM."
By this article, logic would tell me that tightening the spring pressure would give you more bottom end power, but less top; and vice-versa.