Ducati Diavel Workshop Manual page 781

Flywheel - alternator
Lubricate the surface (L) of the driven gear (21) with engine oil.
Install the driven gear on the starter clutch, ensuring it is properly seated.
Note
To facilitate installation, rotate the driven gear in the direction of the green arrow.
Hold the flywheel (17) with one hand and check that the driven gear can rotate freely in the direction of the green arrow
but not in the direction of the red arrow.
If either of these two conditions is not met, this means that the starter clutch has not been installed correctly.