The initial advance is set by hand. The rotor phase is set at that baseline. If you twist the dizzy, you are changing the rotor phase. Since we need a spark event sooner as rpm rises, and we cant reach out there and give another twist, the mechanical inside does the job.

Why else would mech advance be there? No way something that elaborate was conceieved and installed in a trillion cars otherwise. The vac advance does the same job, but needed a third way of changing rotor phase based on load.

It doesnt follow the pickup or ecu or anything else is determining when to switch on to give a spark event sooner. That is the only way you could get a spark sooner for the mech portion of the program, if the idea is the rotor phase isnt alerted by the mech advance. Im talking strictly about stock parts here, no multispark, crank triggers, fixed distributors, etc.