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I am not personally familiar with it, but have heard good things about it from many yoga teachers (such as Kathryn Bruni-Young). From what little I know, it is nose-based breathing (of which I am a fan) and avoids "over-breathing" when not necessary. Sounds yin-like to me!
I was recently made aware of this book by Patrick McKeown's book The Oxygen Advantage. Josh Summers told me about it. The author was trained by Buteyko directly so it may be a good resource to learn about the Buteyko breathing style.