Use this tool to check and correct the exact geometric position/angle of the rear mech hanger in relation the frame dropout. In order to do this, first unscrew the derailleur and mount the rear wheel in the frame.
1. Remove the nut from the threaded spindle, then screw the tool into the derailleur hanger as far as it will go and lock it with the nut (SW 17).
2. Check at several points that the perimeter of the wheel is evenly spaced from the rim (using the tool measuring pin).
If there are any differences, the derailleur hanger position needs to be adjusted accordingly, by applying gentle pressure on the tool to bend it back straight.