The cultivation of Dan (丹) is a milestone of the practice of Taoist Meditation (or Taoist Yoga or NeiDan practice). And dantian (丹田) literally means "the field where dan is cultivated"! According to the text of Hui Ming Jing, the cultivation is a two step process: microcosmic circulation and macrocosmic circulation. The former used more mind-power and physical effort (有為) whereas the latter used minimum mind-power (無為).
The practice of which, in particular the latter, needs much guided effort and patience. Much detail of the text had been used to explain them. Interested readers should refer to the text itself for guidance.
What I'm interested to discuss here is "what is dan", assuming that it has been produced or in the process of final production? And what benefits can one get after reaching this state? And what is the next step of the practice?
Dan is in its essence a concentrated existence of chi. It is the hub of a wheel, and the existence of which depends on the spokes. Dao De Jing metaphorically called the hub Tao which looks like nothingness but is powerful enough to be the essence of a wheel, but whose existence depends on the physical existence of the spokes. Dan is similar. Its existence depends on the "spokes". When fully developed, dan is exactly the central point that balancing out all the chi-connections to the whole body. When these chi-connections are there, dan is there. And when these chi-connections are getting more and more (almost like infinity, because in theory, every cells, molecules of the body can be energized to be balanced out through the dan) or when the chi-connections are getting stronger and stronger, the dan grows! And mentally, the practitioner can feel the existence of Dao itself (the hub as mentioned in Dao De Jing is created). And the physical heath benefit is obvious, because all the chi-channels of the body, with the tendons and bones are strengthened/balanced/tuned in the process.
And for spiritual practice, an "embryo" (a metaphor) is formed. How can it leave the body? The theory is actually not complicated. One only needs to "open" the top of one's head (百匯), and let the connection with "North Star" be another spoke of the wheel!
For a modern man who is satisfied with a good physical health, a calm mind, a good EQ, and with a taste of Tao, he would most likely be satisfied with the satisfactory completion of microcosmic circulation and perhaps with the more ambitious target of forming the dan. Embryo forming and subsequent joining the Universal Tao have better left to the most devoted Taoists.
And I rest my case.