The power piston opens (extended) when there is low vacuum at the manifold port on the base of the carb. It offseats a little ball valve which bypasses the main jet allowing more fuel in the mixture through a drilled oriface. As vacuum comes back up, the valve retracts and lets the ball re-seat, leaning out the mixture for cruising. The power valve is absolutely necessary to get good fuel economy and have a drivable engine.
