Inner alchemy (Neidan or Taoist meditation) qualifies as esoteric practice for the simple fact that every claim of physical and mental change happen inside a practitioner's body - ONLY through being seated and meditate!
On a previous post I looked at the practice from an energy point of view (Entropy and Taoist Meditation). On the physical or physiological level, what are the changes?
When the mind is focused on the point of the third eye and on the point at the lower abdomen at the same time, the latter, through the breathing mechanism and the combined effect of the two focused points, acts as a handle for the mind to control the movement of the pelvic floor muscles. Through meditation in this manner, the pelvic floor muscles will be controlled by one's focused mind with energy channeled towards the control point at the lower abdomen. In terms of physiology, we have arrived at a situation in which a practitioner's mind can control the muscles movement of the pelvic floor as well as direct that energy towards a single point at the lower abdomen. Metaphorically, it can be described as the hub (the "empty" point) and its spokes (the rim of the wheel being the pelvic floor muscles). Reference: Tao Te Ching Chapter 11.
When the force (chi) thus generated becomes stronger and stronger, physiologically a practitioner can feel his lower abdomen (下丹田) jumping rhythmically by itself, coupled with a warm (or even hot, depending on individual) feeling. Those who know how to do micro-cosmic circulation will take this opportunity to direct the chi in a reverse matter up the spinal cord (since energizing the pelvic floor area will necessarily also having chi at the genital, Taoist masters in the past metaphorically say "chi running along the usual route will create a new baby whereas chi running in reverse direct will create the (golden) embryo").
Chi ball going up the spinal cord is no simple feat. Some preparatory tasks like point meditation should be practiced (different masters/texts will have different techniques, it is here the controversial sexual dual practice comes into the picture, interested readers can refer to my previous (and future) posts on the subject for reference). The physiological concept is actually quite simple. When the chi-ball travel (or made to travel) in reverse direction up the spinal cord, the pelvic floor muscles will be further pulled and energized. And assuming blockages the spinal cord can be cleared (or pre-cleared) through this upward chi thrust), the chi shall be directed back to the abdomen (丹田) via the head, the throat etc (任督二脈).
There is one further trick here. Now that the chi-ball will be closer to the diaphragm than to the pelvic floor, the practitioner should make use of this good opportunity to exercise the diaphragm through breathing (please note: reverse abdominal breathing all along).
This is a very important point here. For those practitioners only do the preliminary stage of chi meditation without reverting the chi flow using micro-cosmic circulation, they can only exercise their pelvic floor muscles and can't exercise their more power muscles of the diaphragm (in short, they can't go any further practice-wise). Breathing of the junior type will necessarily be too shallow for both physical and higher spiritual practice. In terms of physiology, without adequate or proper exercise of the diaphragm, the upper part of the lungs and their related muscles can't be exercised.
What is the next step after exercising one's diaphragm? The next step is, again through deeper meditation, to create a single more powerful chi-ball (or energy center or "hub") so that it can control BOTH the pelvic floor muscles and the diaphragm at the same time! Again, this is no easy task. A highly focused mind is required for the purpose. In this respect, the chi-point originally created at the third eye shall be strengthened and with its mental faculty totally immersed into the chi-ball at the abdomen (凝神入氣穴).
Getting complicated? Not yet finished! The next requirement is to have this mega chi-ball NOT only controlling the pelvic floor muscles and and diaphragm but also the muscles (and internal organs) of the whole body! Sounds ambitious. You bet! In Inner Alchemical lingo, it is called "Clearing the eight psychic channels" required as a separate preparatory practice. A small chapter, chapter three, was devoted this subject in Taoist Yoga (the famous book by Taoist Zhao BiChen 趙避塵, translated by Zen practitioner Charles Luk into English). This is also a stage at which seated-only, more-mind-focused, more-spiritual, more-scholarly practitioners commonly stumbled, blocked and thus can go no further (sad, er?). Clearing the eight psychic channels need a good dosage of more physical practice (still mind-body in nature). Such practices will involve mind-body exercises like zhan zhuang (standing meditation) and tai-chi. If one is familiar with yoga as a complete physical and spiritual practice, one can appreciate that a yogi has to practice his Asana to high degree of proficiency before he can tackle more higher spiritual practices. It is the same for Taoist internal alchemy.
OK, what next? After one can totally control one's mind and body using a single chi-point at one's abdomen, one can lead the chi-body carefully (without that much of conscious effort like when doing the micro-cosmic circulation) to go through the spinal cord again, but in this new perspective. This is call macro-cosmic circulation, because it is meant to include everything, and everything goes by itself just with a minimum of conscious effort (since there is still minimal consciousness on one's body, it is still not yet Tao). Again, different masters and texts offered different help and/or suggestions on the macro-cosmic circulation.
That finally rests my case on the physiology of Inner alchemy. Any stage further will be in the realm of the purely spiritual or psychological domain. And reckless venture into this final/defining area is of course totally at one's own risk: practitioners be warned!