If I apply a given amount of force over a small area, I create more pressure than if I apply the same amount of force over a large area.
This is easily expressed in a simple and common law of physics: Pressure = Force / Area (P = F / A).
For those who don’t like formulas, a simple example can help. If I apply a 100 pounds of force over an area of 100 square inches, I’ve applied a pressure of 1 PSI (pound per square inch). If I instead apply that same 100 pounds of force over one square inch, I’ve applied a pressure of 100 PSI.
Even if we don’t do the math for every application of force, the inherent relationship is intuitive. Even small children follow it easily when I explain it to them. And some applications of it are equally simple. When we strike, we want to make our strike contact point as small as possible. Every karateka knows to hit with the two big knuckles when you punch. It does more damage that way. On the flip side, every judoka knows to splay out as much as you can when you breakfall. It spreads the force out and does less damage to your body. These two very different scenarios are simple applications of the same core principle.
But the principle isn’t just physical in nature. It also applies in warfare. Concentrate your forces and hit your enemy in one spot. It’s much harder for him to defend against you that way. It also applies in social situations. Put a lot of pressure on the weak link of a group and the whole group finds it harder to defend. Or apply it to an individual emotionally. Pick at someone’s sore spot and they’ll break far faster than if you pick at everything.
This is an absolutely fundamental principle of the martial arts. Every student of combat must learn it. But it’s also an absolutely fundamental principle of human dynamics in general. Fail to understand it at your own peril.