There has been quite a bit of discussion regarding propellers and what
drives them from one blade angle to another. The link and text below are
from Hartzell. There is another link to McCauley at the very end of this
post which has good info and nice diagrams.

This information may help to eliminate some confusion. In the event of a
powerplant failure it would be beneficial to know how YOUR
propeller/governor works.  Obviously not all engine failures result in loss
of oil pressure, but knowing what will happen in a given failure mode could
make a world of difference.

Ted Stanley - A&P-IA


http://www.hartzellprop.com/engineering/sitelink_faqs_hp.htm

What is the difference between an oil-to-decrease-pitch and an
oil-to-increase-pitch propeller? Are they interchangeable on a given
installation?

It is not advisable to interchange propeller types on a given installation.
The following explanations show why each propeller type is selected for a
particular aircraft.

For most general aviation propellers, an oil-to-increase pitch propeller is
the simplest, lightest and most affordable form of controllable propeller.
When the prop is rotating, centrifugal force acts not only to pull the blade
firmly against the hub, but also to twist the blade toward low pitch.
Therefore control force only needs to be supplied in one direction - to
increase pitch. The absence or reduction of this control force naturally
results in a reduced pitch. The governor continually monitors the engine
speed. When it senses that the RPM is higher than the setting selected by
the pilot, it supplies oil to a hydraulic cylinder on the front of the
propeller. The high-pressure oil pushes on a piston, which causes the blades
to increase in pitch until the RPM returns to its preset value. When the
governor senses an underspeed, it allows oil to drain from the cylinder,
decreasing blade angle, until the RPM increases the desired amount. A spring
inside the propeller ensures that the blades return to low pitch when the
RPM is too low to generate sufficient centrifugal force. This type of
propeller is used on most single engine airplanes.

Conventional twin-engine airplanes need propellers having the ability to
feather (where the blades are aligned parallel with the direction of flight)
when there is a loss of power or a loss of oil pressure. The oil-to-increase
pitch propeller would present a safety hazard to a conventional twin
airplane because it defaults to a low pitch position in the absence of power
or oil pressure. If power is lost on only one engine, its propeller will
windmill creating negative thrust, which when combined with the positive
thrust from the remaining engine, makes the airplane difficult or even
impossible to control. A feathered propeller has very little drag, so there
is less asymmetric thrust with one propeller feathered than with one
propeller windmilling and the aircraft is more controllable.

For a propeller to default to feather instead of low pitch requires that
both the propeller and the governor be significantly different from that of
the oil-to-increase pitch propeller. Each blade is generally fitted with a
counterweight, which is sized and positioned such that centrifugal force on
the counterweight assembly causes a twisting action toward high pitch. A
large spring is also used to force the blades into the feathered position as
RPM diminishes and the counterweight becomes ineffective. Control oil must
now be supplied to decrease pitch, opposing the forces of the counterweights
and the feathering spring. The governor for such a propeller operates in the
opposite sense from that described previously, supplying oil only if it
senses an underspeed.

Acrobatic airplanes also benefit from using oil-to-decrease pitch
propellers. These propellers are nearly identical to feathering propellers
except that their high pitch angles are limited to values similar to those
used in oil-to-increase pitch props. Due to the attitudes in which acrobatic
airplanes operate oil pressure fluctuations sometimes occur. These
fluctuations not only affect engine lubrication, but also change the speed
of the propeller. If acrobatic aircraft were to use oil-to-increase pitch
propellers, this could result in a momentary overspeed at the same time the
engine is starved for lubricating oil. Using an oil-to-decrease pitch
propeller results in an underspeed in this situation, which helps to protect
both the propeller and the engine.

ALSO GO TO

http://www.mccauley.textron.com/pro-sup/prosupframeset.html

for a similar discussion from McCauley