Yeah, that flex-hone tool.
The friction surface. When I changed brake compound, I did not have it done and they are now a little glazed. Brake still works great, but I know it can work better. For car rotors all you need is the flex-hones tool and the brake lath, but bike rotors are a bit different due to mounting and width.
Just like you said adam, you almost need a rig with some kind of a turn table underneath a drill press with the flex-hone attached to do it proper.
A bunch of racers told me they do it by hand with sand paper and in a specific pattern, but it is a long process to do it right…