A clutch can be balanced on the bench with the right equipment Nick, but you're right to say this isn't usual, as the fitment can also throw the balance even if the clutch is balanced. Many modern clutches are so well balanced when they leave the factory, that further separate balancing isn't necessary. As far as I'm aware the SD program is a process to 'mate' the new clutch and its fitment to the flywheel, rather than to balance the clutch itself. The engagement of the clutch to the flywheel is random so it's a case of matching both components to within defined rotational parameters.