All I know about motor timing is that if you turn the endbell anti-closkwise, you're adding timing. and the other way you're retarding the timing.
Add timing gives you more power, RPM etc.....But it decreases your motor efficiency and life of your comm.
If you cut your comm, the decrease in comm size too increases timing.So you must turn your endbell clockwise a little to compensate for the decrease in comm size.
As for the science part, hope more of the motor tuners here will add in or edit what I've written...