A friend and me had a discussion on the Pre-heaven Skill (Xiantian Gong) a month or so ago. Actually, the discussion was inspired by PJ. PJ created a thread on internal energy healing/regaining lost energy etc.
Reverend Yideng was mentioned and used as an example, his healing of Huang Rong was also compared to Xiao Feng trying to cure A Zhu and A Zi.
Anyways, we discussed that in certain cases of 'internal injuries', there is little damage or damage that is not lethal to the "Yuan Qi."
Ancient Chinese medical science (which is often linked to Taoist internal/external alchemy due to Taoist priests trying to reach immortality and hence greatly contributing to the medical science of the ancient times), yuan qi is the original qi which you have from birth (pre-heaven energy/ life force).
Apparently, it is believed that this congenital qi is a source of considerable power, if one could tap into this source you would be able to accomplish unimaginable feats. Therefore, qi practitioners [煉氣士] (lian qi shi) were also seen as sorcerers, because they were able to perform fantastic things. In fact the term [煉氣士] was synonymous to the term sorcerer/wizard. Some translators have even translated the [煉氣士] to sorcerer. I believe some English translations of the novel [封神演義] (Investiture of the Gods/Creation of the Gods) have translated the term this way.
Back to reverend Yideng, we knew that South Emperor had to use the combination of Yiyang Finger and Pre-heaven skill to heal Huang Rong. We also know that the combination of these two skills were powerful enough to nullify the Toad Stance's stored energy.
So, basically we believed that the pre-heaven skill is a cultivation technique that allowed the practitioner to tap, merge, channel your life force energy with later acquired internal energy. Also the name of the skill [Xiantian Gong] [先天功], Pre-Heaven Skill would point to that this skill should have something to do with using your [xiantian qi/yuan qi].
We based this on how reverend Yideng cured Huang Rong. South Emperor cured Huang Rong, but in the process his yuan qi was severely depleted. We believed that he used the Pre-Heaven Skill to tap into his own life force and used Yiyang Finger to channel it out to replenish the yuan qi of someone else (Huang Rong). Doing that will take a heavy toll on the body, you essentially give up a piece of your own life force up.
Making use of your own yuan qi and merging it with your own later on acquired internal energy is a very difficult task. From what my friend told me about reaching the point of merging and tapping into your yuan qi, it requires a deep and profound level of attainment in your cultivation to do so. Maybe, that is why Zhou Botong and the Seven Masters SEEMINGLY never learnt it. (But we cannot be certain that they never studied this skill). Pre-Heaven Skill is probably a key to doing that, it was always suggested that Central Divinity had the most profound and deepest internal energy.
This trail of thought made us think that the internal energy cultivation techniques of the Xiaoyao School operate in a similar way.
They are more or less famous for their motto: If the energy flows within us, we live. If the energy is gone, we perish too.
We see that Wuya Zi, Tianshan Tonglao and Li Qiushui all died after they suffered from energy depletion. In our opinion, we think that Xiaoyao energy cultivation skills are to use (un)orthodox methods to fuse/merge your yuan qi with later acquired Qi to form a new power source of energy, namely Xiaoyao energy (maybe even Beiming energy).
Xiaoyao School's internal alchemy cultivation is all about messing up your normal qi, breathing flow (see how Tonglao forced Xuzhu to recite those verses on their way to Xixia which were quite difficult to do).
Beiming Shengong was an art to absorb and to merge all the energies into one, Beiming energy.
Xixing Dafa is derived from Beiming Shengong but this art was not as good as the original. Mainly, it failed to have a formula to merge the energies to one. The conflicting energies damaged even the life force of Ren Woxing, Xixing Dafa did not have the technique to merge the outer energies with the practitioner's yuan qi.
The only trouble we encountered was Jiumozhi, he did not die when his internal energy was absorbed by Duan Yu even though Jiumozhi studied Xiaoyao School energy.
We figured that Jiumozhi started his Xiaoyao energy study rather late and it has not fused (entirely) with his own energy. Furthermore, in the new edition we learn that his primary energy basis is the Fiery Blade Skill.
Therefore, we assumed that this was the reason why Jiumozhi survived his ordeal with Duan Yu.
With this we also believed that because some martial artists have not merged their acquired energy with their own congenital energy, they do not die when they lose their internal energy. For instance, Hong Qigong’s acquired energy was almost totally gone after purging out the West Venom’s poison. Furthermore, the blast from Ouyang Feng damaged his yuan qi, even though he was not in mortal danger anymore but Hong Qigong was after that incident even weaker than an average man.
We also think that Jiuyin, Jiuyang, Yijing, and so on are about building up a new energy source and merging it with the existing one you have from birth.
This theory is just a speculation of two Jin Yong fanatics and a lot of is based on my friend’s knowledge on Taoist alchemy.
* I am writing all this nonsense down to distract myself from the fact that in about two months, it's my big day. I just needed to do something that was totally unrelated to that event, I wrote this all in a hurry based on a conversation I had with a friend over a month ago. So, there could be room for improvements.