Recently I wrote about “The Way of the Bow” or “Kyudo” as one of the Japanese martial disciplines that developed out of previous fighting techniques into a path for life. As in all of the “Ways”, there is an emphasis on maintaining awareness of the “Seika Tanden”, that point located a little below the navel and slightly inside the abdomen. O-Sensei frequently referred to this idea in his teaching. While there is nothing anatomical that corresponds to the Tanden, it is a notion found in Yoga and all of the practices that spread from India through China and Korea to Japan.
Developing awareness of this psycho-spiritual focus while training and in daily life is one of the goals of Aikido.
In “Hara: The Vital Centre of Man” (1975 US paperback edition) Karlfried Durkheim, a practitioner of Kyudo, presents a detailed discussion of this concept.
For a brief introduction, see the Wikipedia entries on “Dantian” and “Hara”.
As a Humanist, I emphasize rationality and the scientific method, so I find some parts of the presentations unconvincing. At the same time, after years of Aikido training, I know from experience that seeking “the elixir field” is important and beneficial.