User blog comment:BenRG/Episode 6 - Thoughts and Meditations/@comment-1.122.253.3-20130823131959/@comment-4773581-20130823135959

Kiais (the yell you're talking about) and Ki/Qi/Chi are unrelated. Kiais are a combination of psychological warfare (someone yelling very very loudly at you while punching you is generally scarier than that same someone simply punching you without a sound) and exhaling to tighten your abdominal muscles.