If you search on here, you will find quiet a bit of info on this. What happens in the governor depends on what type of airplane you are flying, either a single or a twin. But, both systems use oil pressure to change the pitch of the blades on the prop.
In the twin, the front of the prop dome has a big spring and nitrogen pressure that is trying to feather the prop. The oil chamber is behind the hub and if you add oil, it will force the blades to a low pitch/high rpm setting.
The single has a large spring behind the hub, that is trying to force the blades to a low pitch/high rpm setting and an oil chamber in the prop dome. If you add oil to the prop on a single, it will go to a higher pitch/low rpm setting.
Knowing this, you can picture what goes on in the governor. It is geared to the crankshaft and the insides of the governor are always spinning around. At the very top of the governor is a pair of 'L' shaped flyweights that are controlled by the speeder spring. If the force exerted by the flyweights is in equilibrium with the speeder spring, the prop RPM is constant.
If you adjust the prop control in a single to a higher RPM setting, you are increasing the tension on the speeder spring and the flyweights will move inward. When this happens, they will push the pilot valve down and let oil out of the prop. This allows the spring behind the hub to push the blades to a lower pitch and the RPM will rise.
The opposite happens if you decrease the RPM. Now, you are decreasing the tension on the speeder spring and it the flyweights will move outward, raising the pilot valve, allowing oil to be pumped into the prop hub, moving the blades to a higher pitch/lower RPM setting.
