I dropped my MacBook Pro (Retina, 15-inch, Mid 2012) on it’s corner. It didn’t cause any internal damage – the computer worked normally – but the the lid (with the display) became misaligned.
The misalignment made the lower right corner of the display scrape against the base of the computer whenever the display was opened. I thought if I could loosen the screws that held the display, I could realign the display with the base of the computer.
Using the tool is straight forward. One end of the tool threads into the the derailleur mounting hole. Using the tool as a lever allows the hanger to be bent back into alignment. There is an adjustable indicator that measures the run-out from the braking surface of the rim. When that distance is equal at four points (measured at 90 and 270 degrees for horizontal alignment and 0 and 180 degrees for vertical alignment), the derailleur hanger is in alignment.
Park Tool’s video on How to Align a Derailleur Hanger Using the DAG-2.2: