The power of a punch does not come from the arms, but from the whole body. Here is what Bruce Lee wrote in his book, “The Tao of Jeet Kune Do.”
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Straight hitting (and straight kicking) is the foundation of scientific fighting skill. It developed late in history and, therefore, is the product of careful thought. Requiring speed and intelligence to use, it travels less distance than round arm blows (or hook and spin kicks) and will reach the mark first. Straight blows (and kicks) are more accurate than hooks and swings and allow full use of the arm (and leg) reach
Straight hitting is based on an understanding of body structure and the value of leverage. It is an attempt to use body weight in every blow, hitting with the body and using the arms as merely the vehicles of force. Arm action alone is insufficient to give real power to blows. Real power, quick, accurate, can be obtained only by shifting the weight in such a manner that the hip and shoulder precede the arm to the center line of the body.
(Bruce Lee, “The Tao of Jeet Kune Do”)