There are two types of martial arts, internal and external. In this article I'm going to talk a bit about the two of them and their difference.
External martial arts can be called "overt". They use external muscular strength to crash into an incoming attack. With this method, bigger muscles are helpful. All martial arts have methods to deal with overt attacks and blocks. Then they do this, I do that. I like to see overt actions like the hours on a clock; 1, 2, 3 and so on. Relatively easy to see and read.
Internal martial arts can be called "covert". They use subtle movements that are difficult to read; their intention will not be obvious and may be outright designed to deceive. When you think they are doing one thing they actually are doing something else. In order to defend against these types of covert attacks, it's necessary to be able to read them and have subtle enough responses that their attack can be managed - without letting them know you are up to something. It's truly a covert game. I like to see covert actions like the smaller divisions on the clock.
Some people can only read minutes. Some people can read seconds. Some people can read 10ths of seconds. Some people's clock is analog and if you move even the tiniest bit they will know. The real skill is to be able to read and deliver energy (attacks and defenses) that is below their level of reading. If they can read only minutes, you can deliver an attack with "seconds" subtlety and they will not be able to read it. Your attack will be met with no resistance.
We call this "listening energy" in internal martial arts - ting jing. If you can listen more acutely than your opponent, you will be able to hear (sense) their attack before it gets started. Let's say their attack takes 1 "hour" to happen but your listening and reaction is at the "seconds" level. You have all the time in the world. The real skill in high level internal martial arts is to improve your listening and delivery to the point where fewer and fewer people can read it.
Add to this the internal martial arts' power delivery system and you have a very formidable method.
The overt systems use raw power to deliver very heavy hits. They surely will do the job if they hit. The covert/internal systems do things a bit differently.
Anti-tank warfare borrowed a page from the internal kung fu playbook. A relatively soft warhead which may not be powerful enough to pierce the tank's armor impacts the outside of the tank. It creates a shock wave that travels through the steel armor and causes a strain fracture to the metal on the inside. The inside of the tank metal essentially explodes and the spall (fragments of the metal armor that are broken off) cause damage. It's similar to a hand grenade going off on the inside of the tank. (look up "spall" or "HESH" anti-tank shells for more information).
The point is that our internal or soft style of kung fu has been using this type of energy for thousands of years. The energy from the hit travels into the body as a compression wave that expands and causes damage on the inside.
At the highest levels, the techniques of martial artists become migrate from over to covert; their techniques change from hard and simple to soft, flowing and complex. As part of this migration the martial artist becomes more difficult to read and can accomplish more with less resistance.
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