2. The SnapThe first thing you learn when you start boxing is the jab. This versatile punch is characterised by its speed of deployment. Many kinds of jabs exist, from the fake jab to the set-up jab, and it's not an exaggeration to say that almost all combo moves in boxing starts off with a jab. As ubiquitous and important it is, it is also true that it is actually extremely difficult to pull off a jab properly.
The correct jab involves what many boxers call 'the snap'. Now, you might be imagining some 'snapping' of the wrist as you thrust your fist forward, but it's not as simple as that.
Before we go on, let me embed a video I made. In it I get Master Oh of 경희복싱 to demonstrate three different ways to jab: two incorrect, and one correct. He first sinks one-twos into the sand bag using the "brute force" jabs; going on to show some one-twos using "speed-only" jabs; and then lastly, he demonstrates the proper one-two punches using the snap jabs.
As you can guess, the first way has power, but is just too slow. Any self-respecting boxer can see those slow jabs coming a mile away and will dodge them with ease. The second kind may be fast, but they carry almost no power; getting hit by those jabs won't do much. The third way, with the 'snap' properly in place, has both speed and power. To be honest, most people (including me) can't tell the difference between the speed jabs and the snap jabs (could you tell from the video above?), but with training you get to appreciate the difference, and it's quite the satisfying feeling to be able to pull off a snap jab properly time after time.
The Physics of the Snap Jab
In your head, divide your body into the following four independently-moving parts: the left fist, the left forearm, the left upper arm, and lastly, the rest of your body (which includes your other arm). Now, the whole point of the "snap" is to transmit the momentum from the rest of your body, through the upper arm and then the forearm, all the way to the fist. To help you see this, picture a long, taut rope that is plucked at one end: the wave packet created by the plucking is transmitted down the rope to the other end. In the same way, as you shift your entire weight to the lead foot, the snap jab transmits this momentum in a wave-like fashion to your fist. This 'wave' motion is the reason why your shoulder and arm must be relaxed and free from unnecessary tension in order to execute the snap properly. Moreover, the snap jab involves a quick push-pull motion; precisely because you need a push and a pull to form a wave that propagates down your arm.
Because your fist is much lighter than your forearm, which is lighter than your upper arm, and which in turn is much lighter than the rest of your body, the speed of the object in question is increased at each subsequent transmission as dictated by the principle of the conservation of momentum. As we go from the body → the upper arm → the forearm → the fist, the decrease in mass has to be compensated for by the increase in speed. When executed properly, this results in your fist moving at an incredible speed just before impact.
Actually, I think we all have an instinctive understanding of this 'snap' movement. With only a little training, most people know how to "crack" a wet towel like a whip. And cracking a whip is nothing less than transmitting the momentum of your the arm all the way down to the tip of the whip, which is so much lighter than your arm that it actually travels faster than the speed of sound, essentially creating a sonic boom just like a Concorde jet in supersonic flight. Amazing.