After having a group conversation with Joan and Hunn:
San Jiao is a system that connects everything, like a web, and it is better not to view its components separately from the whole, although each part has its own function. The function of the San Jiao is to move Yuan Qi and Fluids, and in its Deep San Jiao function it is supported by the Ren Mai channel. Ren Mai is the foundation of life, in contact with the Mandate of Heaven, and the fundamental source of activity itself (Du Mai). Du Mai expresses itself more authentically when there is stillness within the San Jiao and Ren Mai function.
Du Mai is not part of the San Jiao mechanism. It is more related to what we do with our life — how we show up in the world, how we stand, how we act, and how we take our position. The more stillness there is within the (Deep) San Jiao, the more harmoniously Du Mai can function. Very often there is a lack of stillness, and people begin to push themselves through life, which can lead to impairment of Du Mai, especially in the sacroiliac region.
It is worth remembering that SJ 16 is a very good point to treat this area. It works on both Yin and Yang, brings Yang downward and inward, and is effective in cases of imbalance in the lower back due to too much Yang in the head. However, always take care of the Deep San Jiao function as well, as it is the source of movement and action of Du Mai. Treatment of Du Mai alone will not be effective enough.
The internal duct of the San Jiao has a function distinct from the Deep San Jiao, yet it cannot exist separately from the whole San Jiao system.