The art of karate was developed as a martial art. Martial is synonymous with war. To understand the training in karate and its purpose, we must first look at the history of karate and how it came to be. "Karate" literally translated means "empty hand", meaning no weapon. Karate is believed to have come originally from China. A descendent of the art of Kung Fu, Karate or "Te" as it was known was adapted and modified by the Okinawans early in Japan's history. The samurai of mainland Japan ruled over the island of Okinawa. In order to force their rule over the primarily agricultural people of Okinawa, they outlawed all metallic and bladed weapons from the island, leaving the ultimate weapon of the day the "katana" or samurai sword in the hands of only the samurai.
The farmers and agricultural workers of Okinawa in order to protect themselves from the samurai, developed karate and adapted many of their farm tools as effective weapons against the samurai's katana. Many of these weapons we are familiar with to this day as karate weapons: The staff - used for carrying water buckets and loads, the tonfa - used as the grindstone handle for grinding grain - now a standard issue to most police officers, the nunchaku - used for thrashing grain from the stalks, and the kama or sickle used for cutting down the grain.
The ultimate weapon though was the human body. To make the human body an effective weapon, the entire body went thorough a physical transformation through conditioning which made every part of the body a weapon that could be used against an adversary.
The Okinawan's learned that the body responded to impact on the various body parts by reinforcing the bones, ligaments, and tissues of the body. This process was not an overnight one, but one that required diligent, gradual training over several years to accomplish. Through this tempering process, the body became forged into an effective, deadly weapon.
One of the principle weapons in karate is the hand. Through conditioning the hand not only became stronger but also became harder as well. The hand being stronger was now able to transfer all of the energy generated by the body into the hand strike without injuring the hand. The hand now also being harder, allowed for maximum transfer of this energy with minimal absorption of the energy by the bones and tissue of the hand. A comparison can be made by comparing a 2 pound rubber mallet verses an 8 oz ball peen hammer being used to break a brick. Although the 2 pound rubber mallet is able to generate more energy with a strike, the rubber being soft and resilient absorbs much of this energy and it is not transferred to the brick. The brick therefore does not break. The ball peen hammer, although it is much lighter, is able to transfer the full energy of the strike directly to the brick resulting in the brick shattering.
The same holds true in karate and conditioning of the body parts. The body becomes not only stronger and tougher, but harder as well. This maximizes the energy transfer to the opponent thus making the karate blow whether it by with a hand, foot, or elbow devastatingly powerful.
The makiwara, or striking pad, is one of the principle tools used in tempering the hands and feet. It is stricken repeatedly with various parts of the hands and feet. At first few repetitions with relatively soft strikes are used. Over a period of several months and years, the strike can be made harder with greater repetitions. It is important this forging process is done gradually to prevent permanent injury to the body. Going too hard or with too many repetitions is a sure way to injure one's self, perhaps permanently.
Tamashi wari, or breaking, is one of the ways the karateka tests the conditioning of the body. Wooden boards, bricks, roofing tiles, etc. are all popular materials used in tamashi wari. Like the conditioning, the easiest material to break should be used with just a single item broken. This can be gradually increased to find out what the limit of the number of items that can be broken with any given strike. Trying to break too much, too early is a sure way to injure one's self. Again graduality is the key.