In classic Taoist Neidan (yoga or meditation), a practitioner uses sublimed sexual energy to generate the initial chi-power to do the microcosmic circulation. This is the first step of (significant) chi-generation as well as the first milestone to become a Taoist Immortal. Some classic texts challenged practitioners to reach this stage within 100 days (百日筑基)! It is a pity that some current practitioners, who fail to understand the intricacies of the art, falsely believe that microcosmic is the end of everything and wasted years doing inappropriate practice (worse yet, teaching inappropriate stuff if he is a teacher or sifu!), searching and researching, but yet fail to accomplish anything significant on the right path - at the same time claiming that the "proper" way of doing microcosmic circulation takes decades to perfect!
But is this the only path to do meditation chi-kung? The answer is no. The most popular contender is zhan zhuang or standing meditation as taught by Grandmaster Wang Xiangzhai (王薌齋). Nowadays more people practice zhan zhuang than Neidan. For one thing the former is easier to do, and for another thing "kids won't giggle" when learning zhan zhuang!
Anyway, for purely health-seeking purpose, zhan zhuang does have a decided advantage over Neidan. It is interested to note that Wang's prominent student madam Mi JingKe (祕靜克), who is healthily and actively approaching 100, claimed that zhan zhuang's advantage is that it aims at reaching the final goal in one step (一步到位). What exactly does she mean? And to what extend is she correct?
The initial step of Neidan is have subtle control of one's pelvic floor muscles, activate one's sexual energy and sublime it in the form of chi to move along the microcosmic. Bravo, but this is only the first step. When chi is thus circulated, it is not simply allowed to dissipate. It is made to work. How? To use it to train the practitioner to subtly control his diaphragm. That's where Mi's one step challenge comes in.
In zhan zhuang (standing meditation), a practitioner starts with holding his hands in mid-section like embracing a tree. Assuming he practices things correctly, chi generated by his hands and feet (etc) will activate primarily his diaphragm and secondarily his pelvic floor muscles, in addition to other body parts as the practitioner may like to focus on (e.g. through visualization or point meditation). Mi got a very valid point!
Having said that, Neidan does have the following important advantages firstly it can better open one's pelvic floor muscles, secondly its opening of the diaphragm can also be more subtle, and thirdly a practitioner will then be well-positioned to open his third-eye (upper dantian) for spiritual purpose as a next step.