[Christ]

Paul - I'm going to stick my neck out here and say something as I understand it... I might be wrong though.

Advancing the brushes in the motor has to do with the timing of polar transition, relative to the position of the armature... if the armature is spinning really fast, the transition of the polarity of the brush has to happen sooner, so that the momentum of the armature pushes it past the brush as it's reached the new polarity (think: signal latency)

It seems kind of like ignition timing in a ICE - you want to set the timing so that it begins the combustion process as the piston/crank is within X degrees of TDC (depends on engine specs for actual timing) so that it's reaching "critical mass" as the piston/crank is several degrees past TDC (dwell time) to create maximum power from the available expansion of gasses.

That said - You would want to "time" the latency of the signal process when changing the polarity of the brushes so that each polarity transition occurs as the armature is just passing the brush, so that part of the armature's momentum isn't used against the magnetic force...

ideally, you could change the "timing" of the brushes to alter the torque/speed profile of the DC brush motor.

IF YOU'RE NOT FAMILIAR WITH ELECTRIC MOTORS, AND YOU'RE READING THIS FOR EDUCATIONAL PURPOSES, PLEASE DO NOT READ TOO FAR INTO THIS POST. IT IS TO BE CONSIDERED PURELY SPECULATIVE, AS I OPENLY ADMIT THAT I HAVE ABSOLUTELY ZERO IDEA WHETHER IT'S CORRECT OR NOT. I WILL NOT BE HELD LIABLE IF YOU FOLLOW MY INFORMATION AND BLOW SOMETHING UP. BE WARNED.