Mahiro here is practicing the Kankudai (or Kanku Dai depending on which hastily-googled page you're looking at), one of the more advanced routines in karate.

"Muscle fibers do not dictate coordination," Rose says, "and coordination and timing are essential factors behind movements like this one-inch punch."

Because the punch happens over such a short amount of time, Lee has to synchronize each segment of the jab—his twisting hip, extending knees, and thrusting shoulder, elbow, and wrist—with incredible accuracy. Furthermore, each joint in Lee's body has a single moment of peak acceleration, and to get maximum juice out of the move, Lee must layer his movements so that each period of peak acceleration follows the last one instantly.