The notion of three Burners (San Jiao) is one of the diagnostic methods commonly used in Chinese medicine. According to Dr. Li Xin, it predates the diagnosis based on the analysis of organ functions (spleen yang, liver yin, etc.). This concept of three Burners has the advantage of offering a global and dynamic vision of the energy system.
The three Burners are generally considered from an anatomical point of view: an upper, cardio-respiratory Burner, delineated by the diaphragm, a middle, digestive Burner, from the diaphragm to the navel, and a lower Burner linked to the functions of reproduction and elimination. This can be represented as three circles, placed vertically.
The Upper Burner ensures the circulation of fluids.
The Middle Burner feeds the Upper and the Lower Burner.
The Lower Burner vitalizes the UB and the MB; it ensures the elimination of toxins.
However, Dr. Li Xin notes that since our focus is on the energy system, the three Burners can be considered from the angle of their energetic function instead of their anatomical location. They then function as three concentric circles, representing the energetic layers of the body. In this case, the three Burners are not defined by their anatomical location (upper, middle and lower), but rather by their “depth” in the physiology of the energy system. Roughly, we have:
An upper, circulatory Burner (UB), in contact with the outside. It ensures adaptation to, and communication with the environment, and the circulation of fluids (blood, bodily fluids, energy). The organic and energetic supports of the UB are the skin, Wei energy (defensive energy), and the meridians inasmuch as they enable adaptation to the 6 Heavenly Climatic Factors, thus maintaining homeostasis (temperature cold / heat , pressure feng / shu, and hygrometry dampness / dryness); (Tai Yang & Shao Yang layers).
A middle, digestive Burner (MB), whose energy vitalizes both the upper and the lower Burner. The excess energy produced by the MB is stored in the LB. The MB is linked to the notion of “flesh” associated to spleen and earth. It is also related to the muscles, tendons, connective tissue, etc., as well as to blood and energy circulation in the deep luo, and even to the circulation of fluids in the cartilages and the joints (Ying nutritive energy); (Yang Ming & Tai Yin layers).
A lower Burner (LB), which operates as the motor or battery of the body. In addition to its reproductive function, it also enables elimination (driven by the energy provided by the MB) and the crucial storage of yuan energy. In this sense, the LB can be seen as corresponding to the vital organs (kidneys, but also heart, liver, etc.) as their function is preserving life, maintaining the link between body and mind, anchoring incarnation. This is why Consciousness, Spirit, Shen reflect the state of health of the LB: a deficient LB causes the Shen to float, manifesting as mental instability, memory problems, lack of mental clarity, hallucinations, even loss of consciousness or dementia, depending on the severity of the problem; (Shao Yin & Jue Yin layers).
This view of the three Burners introduces the idea of “depth” of the energetic layers and implies an understanding of the flow of energy in the body: Dr. Li Xin speaks of its movements as opening (kai 开), dispersing (san 散) and gathering (he 合, ju 聚 or shou 收), according to context or to the energetic layer addressed.
In a living being, nothing is static. The energy of the three Burners is constantly subject to a cycle of expansion, from the core to the surface, followed by contraction, from the surface back to the core, then expansion again, like an energetic breath. The opening movement is greater in summer and during the day: the expansion phase dominates. It is reduced in winter and at night: the phase of contraction or concentration dominates. Chinese medicine considers that the center, the core, is yin, as opposed to the periphery, the surface, which is yang. In the expansion phase, the energy goes out from the center, yin, and reaches the periphery of the three Burners, the outmost Yang: this opening allows exchange and communication with the outside, as well as the circulation of fluids and energy (upper Burner), and the elimination of toxins. Then the energy, having reached the maximum yang, and enriched by exchanges with the environment, returns from the yang periphery to the yin center, where it gathers to enrich and tonify in turn the core.
UB: Upper Burner, in charge of circulation, skin, vessels, meridians; contact, communication, with and adaptation to the environment; defensive energy Wei Qi.
MB: Middle Burner, in charge of digestion; muscles, conjunctive tissues; tendons, ligaments.
LB: Lower Burner, in charge of elimination; reproduction; preservation of physical functioning and integrity; vital organs.
Arrows: “breathing” movement of the energy of the three Burners. The outward, centrifugal movement is called Yin (blue arrow), as it starts in the deep, yin center of the body and reaches towards the superficial yang. The inward, centripetal movement, is called Yang (red arrow) as it starts in the yang, superficial aspect of the body and returns to the core.
Now obviously, opening the upper Burner or driving the energy towards the periphery promotes circulation, but if the middle or lower Burner are deficient, this expansive movement of the energy risks weakening the system by further "stretching" the middle or lower Burner. Thus, a person with a weak or deficient constitution (Tai Yin Lung, Shao Yin, or Jue Yin phases) should avoid sweating (i.e., hot baths, inducing sweating through herbal medicine, etc.), vigorous massages or violent articular techniques, herbal formulas or acupuncture techniques that move or "open" strongly, intense sports, even excessive social contact (crowd, noise, etc.), depending on the extent of their deficiency.
For this reason, when the lower Burner is empty, the body will naturally emphasize the movement of constriction, of concentration, in order to tonify and replenish the energy system. The person will then tend to withdraw, they will seek calm, isolation, will need rest and sleep in order to let the energy return to the center.
Likewise, when there is sufficient energy in the middle and the lower Burners, it is important to help the energy open to the surface, for example through sport, movement, exchanges with the environment, so as not to create overheating, excessive pressure or stagnation in the center. Care should also be taken to preserve the digestive function of the MB and the elimination function of the LB in order to avoid the deposition of toxins in the tissues (the “flesh” of the MB, & muscles, tendons, joints, Tai Yin Spleen layer), or even the destruction of these tissues leading to pathologies of the vital organs (Shao Yin phase). The digestive function of the MB then takes on all its importance because, in addition to its role in the digestion of toxins (in terms of food quality, but also of emotional "digestion", Yang Ming phase), it is its energy which vitalizes the elimination function of the LB.
The opening and gathering movement of energy in the three Burners is one of the aspects of yin yang and qi dynamics. The energy goes out from the yin center and, like the energy of the Earth rising towards the Heaven, it flows out to the yang periphery. Arrived at maximum yang, the reflux gathers the energy back towards the center, the yin, like the Earthly vapors which, after reaching Heaven, fall back in the form of rain. By analogy, we can therefore consider that the centrifugal movement is the opening movement of yin, while the centripetal movement is the gathering movement of yang. But this is only a convention, because obviously, for an observer placed in the center, the yin movement of expansion "goes" towards the periphery, while if the observer is located on the periphery, this same movement becomes a movement of contraction: the energy “comes” towards the observer, towards the periphery.
The use of the words Yin and Yang can be quite confusing as they are not clearly defined: their usage is fluid and a matter of convention. One could also consider calling yang the "thrust", the driving force of the expansive, opening movement of energy, and yin the resistance opposed by matter. Or speak of yang within yin for the opening movement, and yin within yang for the gathering movement…
It's all a matter of convention: yin and yang are relative terms and not absolute concepts, and can only be defined in respect to one another. Both terms can be used to qualify the same movement of energy, according to the position of the observer! Actually, as many terms used in Chinese medicine, such as the 6 phases (Tai Yang, Yang Ming, etc.) or the 4 levels (Wei, Qi, Ying, Xue), they can be considered as a code, a template, an interpretative framework which we use to understand constantly evolving dynamic processes, and not as normative terms defining static objects.
Beyond the terms of yin and yang energy, the important thing is to recognize the direction of the energy movement in the subject, opening or gathering, in order to accompany the natural healing processes and to make sense of pulse dynamics.
 Traditional Chinese Medicine: Back to the Sources for a Modern Approach, Drs. Li Xin & Claudine Merer, Arbre d’Or ed. 2013
 Huangdi Neijing Chap. 5