One-way bearing
From RC Helicopter Wiki
The central bearing in the main gear assembly, the one-way bearing allows the main gear (and the tail drive gear / pulley) to spin freely in the event of engine or motor cut-off. When under power, the one-way bearing locks the gears together so they spin as one. This allows the main rotor to spin freely during autorotation and as allows them to drive the tail rotor so control is maintained throughout the auto.
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