Kiai is also for taking punches to the mid-section. You meet the blow by slightly tensing your stomach muscles, pushing out to attack the blow, and verbalizing the kiai with a shout from deep in the belly. With enough practice over time, the kiai can be done silently, without conscious thought, enabling a martial artist to take a good deal of physical punishment.
Kiaing to take a strike beats getting gut punched, but if that does happen, a loud kiai from deep in the gut can restore your air.
In the right circumstance, any word or words can serve as a kiai. During my blue belt test, just as I was out of breath, unable to breath, I was told to do another action. I responded by kiaing "YES, SIFU!", restoring my air and ability to breathe and continue the test.