You talk about 'Koshi' as if it is a concept or principle - to most of us the word 'Koshi' simply refers to a physical part of the body, 'hips' specifically - I am an old Ki Society guy and koshi was simply referred to as a hip throw as in koshi-nage; nothing more and nothing less.
Hi Greg -
Since I had never heard of koshi referred to as Graham writes about it in his OP I decided to do a little research. Below are three links to pages where koshi is discussed in terms that go beyond merely referring to koshi as a physical part of the body. It's evident that, at least for some schools of Karate, that koshi is more than just a physical descriptor. It's equated to whole body mechanics, a system for the transference of power in the body and a way that integrates the mid section of the body via multi-directional control of the core. Interesting reading.