Filing is really pretty easy, even by hand.
For best performance, cut the brush width to one half of the comm diameter.
It sounds really simple. What it does is allow the MOST brush wrap while NOT allowing any shorting accross any comm segment.
i have played with this a bit and found mixed results. But it seemed to add a little torque in high RPM motors, but overall power was a bit decreased. But still a tuning option.
Comms last longer though