I'm reading a book by Julian Daizan Skinner called Practical Zen.    He was a former monastic student of Shinzan Roshi, and teaches a sort of hybrid Soto and Rinzai zen.   After running through the basic methods, he starts describing stuff that sounds a lot like internal alchemy. Building the tanden (dantien), developing conditions for the turning of the wheel of law (microcosmic orbit).    He also teaches a type of traditional 'yoga' they used to practice in the temple, that includes stretching as well as ki practices, as well as a sort of qigong meditation.   All of this side of the practice dates back to Hakuin, who learned it from a hermit and revived the Rinzai sect in the 18th century.   All this was fairly surprising to me in terms of its presence in Japanese Zen. I've only been exposed to Soto Zen, which is fairly straightforward in its approach.