Pertronix install vs. wiring diagram

I'll answer simpler. "Rotor phase" means where the rotor tip is when the spark fires. Ideally, it will perfectly align with the copper tower on the distributor cap. It changes slightly with "vacuum advance", but not enough to affect where the spark jumps. If the rotor tip was in-between posts when the spark occurred, it could jump to the wrong post and fire the wrong cylinder.