Body Leverage. Arms generate more power when they are held close to the body. For example, when twisting a jar cap open, you hold the jar in close to your body so you may apply more force to the cap. You do not hold the jar at an arms length when opening the jar. Another example, when pulling a rope, you generate more power if you keep your arms bent with the hands held closer to the body. If your arms are outstretched, you are much weaker. If you pulling an opponent, the closer you are to the opponent the greater your pulling force. When twisting an attacker's wrist, you may apply more force if you hold the wrist close to your body.
Constant Motion. As stated in Newton’s First law of Motion, an object at rest tends to remain at rest and an object in motion tends to remain in motion. This means it takes more strength to move a stationary fist, than it does to keep it in motion. Conversely, it also takes more strength to stop the motion than it does to maintain the motion. So keep your hands moving, not stationary. They will react more quickly if moving. The same principle applies to keeping the body in constant motion.
Flexibility. Flexibility permits muscular strength to be applied fully and safely to the execution of a punch. Increasing flexibility of the entire body permits arms and legs to extend further and increases their range of motion, thus, increasing power. Increased flexibility also reduces the chances of injury by increasing the range of motion of muscles and ligaments, thus reducing the chance of exceeding their limits of motion.