Cleans the commutator. It’s a little honing stone that you inserted where one of the brushes was usually located that cleans off the carbon buildup.

I replied without actually looking at the picture 😅 That looks like a power supply for running the motor during the process. There was a fair bit of “voodoo” regarding motor performance, breaking your motors in, clocking the bell to adjust timing, etc.

It's for testing and tuning motors but I'm not exactly sure how it works.