By shortening the slot on the inside like Rockable did here, then its pretty much a pre-CAP advance. Everything will work as it should without changing springs. Small adjustments on the start can be done by adjusting the spring perches.
I think how and why is pretty well illustrated with Rockable (360 Tune Up) and further discussed with Mullinax."
Advance curves for performance engines using no vac advance (Race) versus when using vac advance. Another timing curve question
