The Motorcraft 4300, 4 barrel carburetor power piston is used to supply extra fuel when powering up.
The power piston is made of brass (at least the piston itself is) and fits into the float bowl top. See #26 in the diagram. The purpose of the power piston is to supply extra fuel when you are in the power mode. At idle and low speeds, vacuum is at the highest level and pulls the power piston up and off of the power jet (closing it). The power jet is located in the bottom of the float bowl. As you gain power, vacuum starts to drop and the spring on the power piston rod forces the power piston down and pushes the pin on the power jet open. This allows more fuel to feed into the carburetor throat.
It is very important that the power piston works smoothly. When you push on the spring and pin end, the piston should move into the top, then when you let go it should snap back. The power piston is moved by vacuum alone, so it needs to work without any binding whatsoever.
You will need to remove the power valve from the well in order to give it a good cleaning. Buff the brass piston using a wire wheel. Polish the well with crocus cloth to make it smooth. Lubricate with Silicon Spray Lubricant.