Ki is the internal life force energy that we all possess. Karate training enables the practitioner to be able to focus this energy into an attack or defensive movement effectively making the movement more powerful than a similar movement without the use of Ki. In karate, the focus of Ki is one of both a physical focus and a mental one as well. Mental focus allows for the entire body to synchronize in movement together with the mind to produce maximum energy generation. The mental training allows the mind to push the body beyond normal limits and allows the body to produce strength and power in the physical body that is normally untapped. It is believed that the center of the body's Ki is in a location called the "tanden" which is located 2 inches below the navel. The Ki flows from this point outward along the body's meridians throughout the body.
Kiai, or the shout used in karate, is one tool the karateka uses to synchronize mind and body at the same time thus generating maximum energy. It is not as some karateka explain, to taunt the opponent. The kiai is a shout that is generated low in the abdomen, not from the voice box. It is accompanied by a quick exhale and the synchronization for a split second of all the muscles needed to produce maximum power in any given technique. Kiai, translated means "to match Ki". What this means is that the spiritual energy and physical energy for a split second are matched to produce power. This generation is typically at the exact time contact is made with the opponent or when maximum power generation needs to be demonstrated and practiced in kata. In kata, this is typically during the last movement of a powerful technique.
Karate training develops the ability to harness this energy. Through the repetition of training, and through physical and mental development, the karateka learns to develop Ki. The development of Ki adds a whole new level of potential to the karate practitioner. It transcends beyond only the physical movement and compounds the energy generation potential of the karateka.