Don't imagine

that a loud shout is necessarily a strong 'kiai'. It must be full of aggression and come from the stomach. Breathing into the stomach rather than (or as well as) into the chest is in fact a rule common to all the oriental disciplines, spiritual or otherwise, that are concerned with breath-control. Breathing in this way does increase lung capacity, but in addition contributes a sense of well-being perhaps physically connected with the resultant lowering of the centre of gravity. One feels more stable - this point is certainly important to karate.