My feeling is the rotor advances CW small block and CCW big block. the reluctor tangs passing the magnetic pickup triggers the spark. When the springs allow the reluctor to advance it gets the reluctor tangs to the magnetic pickup sooner making the spark sooner thus advancing the spark timing.

The Vacuum advance on the other hand moves the plate the magnetic pick up is attached too in the opposite direction also getting the reluctor tang and mag pick up together sooner for their love fest also advancing the timing.

Vacuum advance sucks by the way and should be unhooked in the tuning stages. Yata Yata Yata

oh look hear
http://www.msdignition.com/uploadedFiles...tor_phasing.pdf