The alternator spins at high speed and has a fair bit of inertia. The one way clutch prevents the alternator from backdriving the crank when the crank speed is fluctuating (as it is when the engine is idling.) This stops the belt tensioner from bouncing around. If you are familiar with the Canadian 1.9TD, you may have heard about problems with the crank pulley coming loose due to wear on the keyway. The one way clutch helps prevent this sort of damage by preventing torque reversals on the crank/crank pulley joint. The addition of the one way clutch as well as a slightly different method of "keying" the pulley to the crank seems to have solved the crank pulley problem on the TDI.