Strongarm allows the user to rotate and move their shoulders at extreme speed and power, giving them commensurately fast and strong punches.
Kendo is able to throw a huge flurry of punches that are incredibly quick, and immensely powerful. He was also able to whittle away at Fat Gum, a Pro Hero with a defensive Quirk using the pure shock from his punches.
Strongarm works by constantly rotating Kendo's shoulders at superhuman speeds. Although he appears to ignore it, the drawbacks of his Quirk can tire out his muscles if overused. This power can also only be used for a few seconds at a time. In terms of raw power, it is unable to break through Hekiji Tengai's Barrier Quirk.
- My Hero Academia Manga: Vol. 16, Omake