This's trimming rod helps align the brush hood and tune brush damper, or push brush during motor break-in.
By pushing the damper down gently with this tool the brushes will move freely without bending.
During the motor break-in process you can push the brushes with this rod to sit them quickly, allowing you to be able to the read the correct amp draw for fine tuning the timing.
The other side has a tool for standard 5×4mm brush holders and its length is designed to allow the fine tuning of the both brush holders independently.