One mythical aspect of Neidan is that in some advanced stage, a practitioner has to carefully nurture his "embryo" (the "official period" is 10 months, same as bearing a child) till the day it grows up and ready to leave the physical body. Let's put aside the practice details as well as the mythical part for the moment and look at its underlining philosophy. We can notice one important aspect applicable to all levels of chi-related practices: Chi has to be carefully nurtured.
I had a student (an elderly man) who couldn't walk and had a weakened heart. I taught him zhan zhuang and he did it diligently. In a couple of months' time, he could walk with a cane and with a stronger heart. He believed he was cured, became very slack in his practice and within a few months, he found it difficult to walk, and his heart became weakened again. He was scared and picked up his practice again (already forgotten much I had taught him before, partly due to his old age, partly due to slack in practice). And again with diligent practice for a few months, he started to be able to walk nicely again, and with a stronger heart too!
The thing is zhan zhang, tai-chi and Taoist meditation (Neidan) is not something like a treatment program or a course of "antibiotics". To jump start one's practice, one has to do it diligently, i.e. nurture (and treasure) one's budding chi like nurturing (and treasuring) a woman's embryo. And to keep up (and grow) one's chi, one need to nurture it again! In other words, it is a life-style change. An exercise for the mind, for the body, and for the mind-body. A "habit" that we take, by our own volition, for the rest of our lives. Everyone has a choice.