Trap Flex allows the user to flex their trapezius muscle to extreme degrees, allowing them to cover their whole head with their shoulders. The flexing gives them a fair amount of speed and strength.
While Trap Flex does provide extreme speed and strength, it is nowhere near as strong as other strength-based Quirks, such as One For All, as All Might was able to knock Habit out with a single blow to the back of the head.
- Trap Flex is also referred to as Oversized Trapezius in My Hero Academia: The Strongest Hero's combat archive.
- Trapezius - Wikipedia article on the muscle this Quirk is based on.