I really don't know about what Nagamine meant. I am curious about the sides vs. back for power generation. I always try to generate power "from the floor up". So I guess I would fall into the "back" power generation category, although I have never really studied myself in that manner.

I guess I am not seeing the benefit of using your "side" (lats? obliques? intercostals?) muscles over your back muscles. Isn't that going to limit power generation somewhat? How is this typically done in karate styles?
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"In case you ever wondered what it's like to be knocked out, it's like waking up from a nightmare only to discover it wasn't a dream." -Forrest Griffin