BIRK,
Excellent tutorial here:
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If you lost engine oil pressure in a 172 with a constant speed propeller, the propeller would not go into low pitch right away. The mechanism for propeller pitch changes is a lot like a car jack, supply pressure to lift, trap pressure to hold, let pressure out to drop. So initially when pressure was lost, and before the engine started making serious metal, oil trapped in the hub would hold the pitch in place, assuming you were just flying straight & level, making no changes to demands of power or propeller pitch. As the engine started to slow down due to impending failure, the govenor would act to let oil out of the hub. The CF and the internal spring forces would push the propeller back to lower pitch. When enough load was removed from the propeller to compensate for the additional load due to no lubrication, the govenor would have the valve closed again and the propeller would stay where it is. Ultimately with a real loss of oil pressure the only way the propeller is going to move is back toward flat pitch, there is no going back up toward feather.
-- IFMU