The front of the body is considered to be the
conception vessel and is associated with the
yin, while the back is considered to be the
governing vessel and is associated with the
yang.
A popular martial arts story claims that some years ago, a young upstart martial artist kept harassing an old master, desirous of a challenge. The old master ignored the young man until one day when he challenged him publically, in a restaurant. Since the challenge was proper, and issued in public, it could not be refused. Even though the master was in his 70s, he had to accept.
As the young man prepared to execute his attack, the old master planted into a forward bow, delivering a palm strike to the young man's abdomen. His chi control was so great that in that instant, not only did he end the fight, but he also ended the young man's ability to walk. The move was referred to as the dien mak.
The point of the story is that chi is a force that can be directed outward as well as inward. The advanced martial artist learns to channel it, as a snake channels venom through its fangs. This is a far cry from standard street fighting techniques, where a punch is accomplished by applying full power to the muscle groups that move the arm all the way through the swing. Good martial artists learn that this is somewhat wasteful of energy. They will actually relax to varying degrees through a punch, kick, block, etc., and then abruptly apply full power at just the right quarter-second to deliver maximum damage. Sometimes this involves a snapping or whipping action, which adds additional torque to the move. (Shotokan may be an exception to this, but I'm not sure. Feel free to add a writeup or msg me.) This is an example of proper chi usage - you aren't just hitting someone, you are channelling energy into them.
By the way, chi is pronounced "chee" or "kee" when talking about Qi or whatever the Chinese call it. It is pronounced "kai" when talking about the Greek letter resembling an 'x'.