This article comes from Tiger Kim’s martial arts webpage
WHY WE SHOUT WHILE PERFORMING TAE KWON DO MOVEMENTS
KI-HAP means yell, or outcry; it is a quick, sharp, yell performed by exhaling strongly from the lower stomach, utilizing your diaphragm. Kihaping has a number of purposes. When you yell, you force yourself to exhale, for that reason, kihaping helps to regulate your breathing. Oftentimes when exercising, people hold their breath; kihaping ensures that you breath properly thereby increasing your endurance and stamina. Additionally, kihaping as you kick or punch allows you to remain relaxed and more flexible, thus improving adaptability and speed. To prove to yourself that kihaping allows you to be more relaxed and flexible, try this simple exercise. Stand with your feet a little wider than shoulder distance apart and reach for the floor (with your knees straight), now exhale, or kihap and see how much lower you can go.
Kihaping is also a way of focusing your mental concentration; your shout helps to prepare your mind and body for the skill which you are performing. Your kihap signifies the bringing together of your body, mind and spirit. It is an expression of your personality, your confidence, and your feelings. For that reason, you will find that no two people’s kihaps are exactly identical.
It may also be the case that your kihap will vary depending on the particular skill you are performing. Your form kihap may be different than the kihap you use during sparring. The kihap you use for one kick that you feel very confident throwing may be different than the kihap you use for a more difficult technique you are having trouble with.
For the self defense aspect of Tae Kwon Do, your kihap is always your first line of self defense. Before resorting to kicks or punches, you can use your voice, your kihap, to draw attention to your situation. Being comfortable yelling, and using your voice to stand up for yourself is a self defense skill that you can work on and improve upon from your very first day of Tae Kwon Do training .