The kihap (pronounced "kee-awp") is a martial arts technique found in almost every existing branch. Quite simply, the kihap is a quick expulsion of air from the lungs usually accompanied by a yell of some sort. In general, the kihap is to be performed while attacking or while absorbing a blow (or both). While many people outside the martial arts raise an eyebrow or snicker upon hearing the kihap, it is quite practical in many instances.
The most easily grasped effect of the kihap is that of intimidation. A short yell can easily cause a slight hesitation which is more than enough to allow the martial artist to take advantage of the situation and do something nasty like dislocate someone's shoulder.
However, the more important aspects of the kihap are to be found in human physiology.
The first physiological benefit of the kihap is defensive in nature. When you expel the air in such a manner, not only do you tighten your abdominal muscles against attack, but you are also bringing your organs in tighter and closer together. This helps avoid internal injury. Even if yelling while attacking, those defensive attributes remain intact in case of a counter-attack.
The second important benefit is striking power. If you exhale while striking, the strike is more powerful. This isn't limited to the martial arts. This technique is used in practically every sport. Just think of how a weightlifter synchronizes his lifting and breathing.
As an addendum, I am posting an excerpt from - http://www.shihanryu.org/resources/health.htm
The attacking "yell" that is taught also has its basis in basic physiology. Aside from serving to demoralize the opponent, the "yell" serves also to tighten the lower abdominal muscles to prevent injury in the event of unexpected counter-attack. In addition, the exhalation, or thoracic grunt as practiced also by weightlifters or wrestlers, serves to equalize the pressure increase in the thorax which may result from violent, exertion, thus preventing injury to the vital organs. The complete exhalation of the "yell" serves to expel the tidal air of the lungs, thus increasing the breathing or vital capacity of the lungs.