In order to change the centrifugal advance curve, the springs are swapped. For an MSD distributor, light silver springs provide the quickest rate of advance, while heavy silver springs have the slowest rate of advance. Keep in mind, it's entirely possible to mix and match springs. For example, you might be able to use one heavy silver and one light silver spring in your combination. As mentioned in the text, I've seen some combinations that mandated one spring only (simply because a super-light spring wasn't available).