When the crew commands the propeller to feather, oil flows out of the piston allowing the feathering spring to force the blades towards a feather position.

Full feather is about 89 degrees blade angle. The crew feathers the propellers by activating the engine stop-and-feather control in the cockpit.  This stops fuel to the engine (stopping engine rotation) and allows the propeller governor to vent oil to the engine sump; increasing the blade angle towards feather. With oil departing from the piston, the feathering spring and counterweights can move the propeller blades into the feathered position.