Definition
A feathering propeller design in which engine oil pressure is used to move the blades toward a lower (flatter) blade angle, while springs, counterweights, and compressed air or nitrogen drive the blades toward higher pitch and into the feathered position. Loss of oil pressure therefore causes the blades to move toward feather rather than toward fine pitch.
Plain English
It's a propeller setup where oil pressure pushes the blades flatter, and springs or counterweights push them steeper. If the oil pressure fails, the blades automatically swing toward the feathered (knife-edge) position instead of flattening out.
Context Anchor
Seen in discussions of feathering propellers, especially on airplanes where the propeller must be able to move toward feather if oil pressure is released or lost.
Derivation
The name describes exactly what the oil pressure does in this design: it acts to decrease pitch (move the blades toward a flatter angle). The opposite design, oil-pressure-to-increase-pitch, uses oil pressure to move the blades toward higher pitch. Naming the system by what oil pressure does makes the failure behavior easy to predict.
Why Pilots Care
Loss of oil pressure allows counterweights or springs to drive the blades to the feathered position, eliminating windmilling drag on a failed engine.
Grounding Statement
Picture oil pressure holding the blades out of feather; when that pressure is taken away, the blades are free to turn toward the low-drag feathered position.
Intuition Check
Pitch here does not mean the airplane nose moving up or down. It means the angle of the propeller blades; decreasing pitch means the blades take a smaller bite of air.
Example Sentence 1
Because the airplane uses an oil-pressure-to-decrease-pitch system, a loss of engine oil pressure allows the springs and counterweights to drive the blades toward the feathered position.
Example Sentence 2
On engine failure, the loss of oil pressure to decrease pitch lets the feathering mechanism increase pitch until the blades stop rotating.