[Gaz64]

The point of the check is to ensure the rotor button is aligned with any post at its firing point.

As the advance mechanism operates it moves the reluctor (trigger wheel) AND the rotor forward in timing (ignition advancing) in relation to where the engine is at that point.

The rotor button will still be in the same place with a timing light regardless of the timing, be it 10 BTDC or 35 BTDC etc.

Only changes position on vacuum advance distributors.