TOOPRE Bicycle Derailleur Hanger Alignment Gauge Professional Tool Measure Straighten Dropout Repair DAG-2.2 MTB Road Bike Tools

The reviewer discusses a derailleur hanger alignment gauge, which is a TOOPRE tool, multiple times throughout the video. They state they have used it almost daily for nearly four years and find it fantastic. They demonstrate its use by showing how to check and straighten a derailleur hanger, explaining the principle of using an elastic band to compare alignment at the top and bottom of the wheel. They mention it is exactly the same as any other derailleur hanger alignment gauge and is suitable for both aluminum and other materials, with a caution about aluminum work-hardening after one straightening.