I recommend any articles written by George Donahue, also of Kishaba Juku Shorin-Ryu. You can find several of his articles at the FightingArts.com website. About 10 years ago, George was very kind to introduce me to koshi and Kishaba Juku body dynamics. Please see:
I like his recent article, Koshi/Yao: An Introduction.
Charles C. Goodin