Leverage off the back foot, hips, body, setting down on your punches. It's difficult to verbalize, one picture is worth a thousand words.
It comes from the proper alignment and usage of the legs, hips, shoulders, and arms. It comes from balance and timing. Illustration: Joe Louis. Just look at the power on the heavy bag at the begginging of this video. [yt]ZR9ty29k6ZE[/yt]
It comes from a couple places, obviously your legs, but the snap comes from your forearm/Wrist, thats why I believe every young boxer should invest in a wrist roller, I used to do countless sets of them when I used to box, it really helped me with snap at the end of my punch, especially in my jab and straight right.
I was right too..lol the corkscrew/whipped action in the shoulder is just very basic..Hearns got his power from this whipping effect. The legs, Pacquaio, Tyson are great examples..Huge legs
The transfer through the waste is very important. Great punchers also understand extension. They place their punches strategicaly so that their opponents recieve the full force of the transfer of their weight.
Its also fair to mention that while power is derived from these basic fundamentals, fighters deliver their power in different ways. Hearns was a master of measuring a fighter and extending his right hand through his target. If I could make an analogy: A baseball player hits the ball the hardest at a critical junture in which his swing has reached its full force, meaning just after his swing is initiated and just before his swing crosses the plate and starts to lose momentum. A boxer does the same, as his punch needs momentum to gather force, but eventualy begins to lose this momentum, Hearns is a perfect example, his right hand would gather its full momentum and at the peak of its motion, would hit his opponent. A few inches shorter, the punch is smothered, and a few inches further, the punch would lose momentum. This is a function of measurement, timing, extension, and perfect technique. Pac on the other hand garnishes power through the movement of his entire body, his punches (especialy the straight left) have his entire body weight in motion behind them. This is a devastating way to deliver a punch and also leaves a fighter vulnerable if he misses.
True punchers are guys who have the god-given ability to go from pre-stretching their muscles to contracting their muscles at a much faster rate than others. Leverage, timing, locking, and sitting on punches further add to punching power, but the true phenoms like Julian Jackson or Tito Trinidad just have muscle firing patterns that generate muscle contractions quicker to produce enormous power in a very short period of time.