The practice of chi-kung and Neidan have always been viewed with some degree of esotericism. Are they really esoteric, and if yes, how is it like?
Practitioners of hard chi-kung (a stream of martial art practitioners 氣硬功) like to practice breaking hard or pointed objects using one's weakest body parts, like throat or head (cranium). And some Tai-chi practitioners like to demonstrate uplifting the body of his partner by seemingly a gentle push. With chi-kung getting more popular and its mechanism being understood, the above demonstrations don't seem to be too esoteric nowadays, and demonstrators are viewed as seasoned chi-kung masters rather than humans with magical power (for example, a demonstrator, without adequate time to generate the necessary chi, will be harmed if his opponent strikes his throat in combat, and a totally relaxed body can neutralize the stunning effect of a tai-chi uplifting).
One of the most esoteric, when viewed with today's eyes, is genital (chi) weight-lifting as documented (and practiced) by "modern Taoist" master Mantak Chia. In his book "Bone Marrow chi kung", he gave a detailed descriptions of the practice (with necessary legal disclaimer that the practice has to be learned safely from his academy), plus the information that necessary tools can be purchased from his school! Its efficacy is beyond any outsider's speculation, but fair to say, even assuming the claimed benefits can be achieved, there are certainly other ways to achieve similar objectives. Besides, unlike the hard chi-kung skills, one can't demonstrate that stuff in public to earn some extra income! Having said that I consider Mantak Chi wonderful in sharing this esoteric practice to the public at large, at least it will not be further mystified in future, like some of the practice that I am going to talk about.
In advanced Neidan, there was a stage in the practice called "馬陰藏相，斬赤龍" (Withdrawal of one's genital, no more menstruation). Pretty scary concept! No male or female will fancy that for whatever benefits. Yet, the Taoist classics are adamant and serious. Recently I came across a Taoist/Tantric website run by a school of reputable teachers. And in the website, they say their advanced practice literally includes teaching people to do "馬陰藏相，斬赤龍" (Withdrawal of one's genital, no more menstruation). They further proclaim one of the benefits being birth control (and the beauty of the practice is that a female practitioner can switch it one and off according to her practice routine!). Can't go more esoteric than that, you might say.
Is there a problem with the classic texts? The answer is a definite NO. What the classic texts meant is a metaphor, and unsuspecting practitioners treat it as physical reality (for example, physical as hard chi-kung). During pelvic floor meditation, the genital will unavoidably be stimulated (that's why some meditators would experience erection during meditation with no erotic thought). When one progresses, one's chi-channels will be opened more and more; finally any chi generated around the genitals will be quickly dissipated along the body's, now unblocked, chi-channels. Some Taoist texts went a little bit further, and told us that after that a practitioner would never have erection forever! (well, if we add, during meditation, that won't be so scary after all).
The puzzle has not been solved completely. You might ask: why all those stupid metaphors, why didn't they spell it out clearly? Carl Jung's concept of psychic reality is the missing link. For the practitioners, these were their psychic reality. They had the belief (supported by certain internal perceptions) that they were then conserving their sexual energy and were using it to stimulate and "cook up" the cosmic energy for eventual achievement of Immortality. The physical fact of dissipation of chi away from the genital was a simple physical fact, but the psychic reality built upon this simple fact could have a huge uplifting/directional/motivational effect on a practitioner's mental and physical energy. In short, it was the power of a practitioner's physic reality that played the trick!
Finally, the challenge for the modern man: Can we can benefit from these classic teachings using the concept of psychic reality instead of mystical belief system?