The spray clutch is basically like the sprocket on the back wheel of a bicycle. Think of the chain being a belt to the engine, instead of to the peddles, and the rear wheel, the rotors and tail rotor. On a bicycle, if you stop peddling, the rear wheel will keep spinning with little friction from the fact that the peddles aren't turning. Similarly if the engine on the R22 stops, you want the blades to have no friction from the engine - those blades will slow down quite quickly enough all by themselves ;o)