Definitions:
Fly weights: The two weights on the image that are near the top painted in red, on the left bend inward and on the right bent outward. These weights are attached to a spinning shaft which is used the primary way an under or overspeeding situation is recognized.
Pilot Valve: The valve shown in the picture that is directing oil into or out of the system. This valve is attached to the speeder fly weights and therefore moves up or down based on the fly weights.
Speeder Spring: The spring on the top of the picture, adjusted by the pilot through the prop lever. The tension of that spring is what the pilot is setting.
Multis Versus Singles
Think about it logically, what would you want to happen if you lost oil pressure in, for instance in the event of an engine failure. If it was in a single engine you would like the prop to keep windmilling giving you a chance to restart it. In a multi engine you have another engine running so going to feather would be the optimal result.
For the above reason in a single engine aircraft oil into the system is used to increase the blade angle so in the event of oil pressure loss the blades go to the flattest pitch. On the other hand in a multi oil into the system puts the blades to a flatter pitch so in the event of a loss in oil pressure the blades go to feather.
Scenarios
Here is what will happen in an overspeed situation: the flyweights will go outward due to excess centrifugal force, the pilot valve will be lifted, and in a multi engine the oil will be allowed to flow out of the system. The result will be the blades pitch increasing preventing the RPM from going up and the pilot valve and flyweights returning to their neutral position.
In an under-speed situation the flyweights fall inward, the pilot valve is lowered, and in a multi the oil will flow into the system. The result is the blade pitch decreasing, keeping the RPM from decreasing and again the pilot valve and flyweights return to their neutral position.
Feather Lock
I am unfamiliar with other multis asside from the Seminole as that is what I flew, so this description will be copied from their manual:
A feather lock, operated by centrifugal force, prevents feathering during engine shutdown by making it impossible to feather any time the engine speed falls below 950 RPM. For this reason, when airborne, and the pilot wishes to feather a propeller to save an engine, he must be sure to move the propeller control into the feather position before the engine speed drops below 950 RPM.
Unfeathering
This can be done through a variety of systems. The two I am familiar with is stored oil pressure and nitrogen systems. Both systems act in the same manner, when the pilot pushes the prop out of feather the stored oil pressure or nitrogen is used to take the prop out of feather for restart.