As a high school teacher who often lectures, I have to project my voice, as someone on stage does, speaking with inflection (monotone can ruin the most interesting material to present), and repeating this in more than one class, more than one day in a row. There is no way to do this if I "project" through the upper chest; it's just shouting. I have to project through the diaphragm, or I'll just strain my voice and tire myself out for the next class. That's a self-inflicted health imbalance to me. Look at a sleeping baby and see how nature has the baby breathe through the diaphragm/upper abdomen, the floating ribs moving with the expansion and contraction of that central area. (Watch out for babies that are awake and crying. Wow! That is one piercing sound coming from such a tiny person.) Singers use the diaphragm as well; that's the core area. My instructor's teacher has visited the dojang on a number of occasions, and she has run us through proper breathing not only when practicing hyungs, but also when sparring and, of course, for the kihap. She emphasizes the kihap as coming from the diaphragm. When I did Tae Kwon Do many years ago, the instructors had no particular yell, such as "Kihap," just that we yell. It was natural for me to yell something like "Hawh!"; it was a long loud shout, though, like "Haaawwwwhhhh!" I was kidded that I was scaring the h___ out of everybody, but my instructors had no problem with it. Comparing sounds, just yelling "Yah!" involves the diaphragm better than another sound/word, such as the example of "Pizza!" That "p" sound is holding your air back; another letter to start with that would be more natural would be an "h" or a "k." I took some cardio-kickboxing classes recently, and the instructor, who had studied Isshinryu when younger, had us yell "Ka!" at certain points. Personally, I yell "Kihap!" at appropriate times because it's approved, and it comes from the diaphragm, but if I felt it would be accepted, I'd avoid the two-syllable "Ki-hap" and choose a one-syllable, easy-starting, vowel-ending yell. The core of the different yell? The diaphragm.