Using the power of math and science, Vsauce determined that an average-sized fist performing a Rising Dragon Fist would apply enough force to decapitate a normal human being. Then they built a dummy to test their work.
You always see gotta-go-fast superheroes punching people when they fight bad guys. And, yeah, a fist delivered at the speed of sound will probably do the job of crushing evil. But a guy like Quicksilver doesn’t even need to hit a person to mess ‘em up real bad.