Just guessing here, but ...

[Harmon Sommer]

My Guess: Electronic ignition timing is done by phase locking an oscillator
to a timing point on the crank. This enables the controller to determine
where the crank is at any  time (dependent upon the freq of the
oscillator)to what ever degree of rotation is desired. This information can
then be used to set spark advance for engine operating conditions.

Or have I guessed wrong.

harmons at sequent.com (Harmon Sommer)