Ki is something of a mystical term for a rather simple physical process. The controlled exahaltion and kiai in the "ki punch," is simply a modification of a grunt. If you've ever lifted something heavy and grunted or groaned while doing it you've used "ki." Studies have shown that grunting actually increase the ability to lift by something like 20%.* So, mystical ki force is BS. Controlled exhalation at the right time does actually have some effect.
*this may be slightly off as it's from memory
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