[X] Her oldest active art, which had seen her through so many trials Forgotten Vale Melody
Cai Renxiang let out a dissatisfied hum, but did not admonish her. Ling Qi took it as a tentatively good sign. Since that day at Zhengui's hill, her liege had… not relaxed per say, but had been a little more permissive in private. "At the most base mechanical level, it is an exercise in spiritual surgery, transferring an art and the meridians it occupies to new housing. It is permanent, and so it is wise to think hard on what is being sacrificed."
Ling Qi looked worriedly at the blade. That was a troubling thought. "And what is the point of that?"
"It trains the mind for the stages which come after," Cai Renxiang replied calmly, letting her eyes drift shut again. "A cultivator is not merely their body, and a cultivator's body is not merely flesh. Even if you can speak the words, you do not understand them without experience. These exercises replace much trial and error which our ancestors needed perform to ascend the realms."
Ling Qi understood. "You called that explanation base and mechanical. Is there something more to it?" She asked, and she felt Sixiang chuckle.
Cai Renxiang was silent for a time, but eventually she answered. "It is not possible to excise and transfer a piece of ones spirit to a new bodily vessel without affecting the self. You know that climbing the realms of cultivation requires sacrifices, things cast aside. What you place in your domain weapon is something which will remain with you always."
Okay so we actually knew about this. Its in our lecture:
"I see the question in your minds, those of you with lesser backgrounds. For the vast majority of cultivators, only the lower dantian is relevant, and so common parlance, it is simply referred to in the singular. You however, are among those for whom the others may be relevant. The middle dantian," he began tapping a spindly finger against his chest. "Is opened in the fourth realm, and acts as a forge to refine your qi into a purer form, known as shen, in doing you will sever your mortal vulnerabilities one by one, and their vital functions will be incorporated into the middle dantian as energy constructs referred to as pearls."
The 'trial and error' method is because you
need to perform spiritual surgery. You're literally severing the function of your respiratory system, circulatory system, immune system, digestive system, and nervous system from your meatbag.
The Domain Weapon is a massive boon in advancement in this case, because it lets you practice.
Severing a Domain Insight Art in this way is vastly safer than severing a vital system. If you fail you need to rebuild your domain to repair the damage, but if you fail in severing your core bodily functions you'd just die right there. Being able to perform this act on an Art gives you the first step of pearl formation.
Its the same reason you aren't recommended to start with a Heart meridian, once you accomplish it, you can do it reliably, but since you're manipulating an intangible, if you fuck up theres no fixing it and theres no way to know you aren't doing it wrong until you do it.
Having a tangible external focus serves as a crutch, the Domain Weapon is an extension of your self. This is less of a leap than you'd think, humans are already naturally inclined to integrate tools into their self image(its one reason why road rage is so bad, people consider their car to be as much a part of them as their leg while driving), but this goes from "tool operated by my physical body" to "tool operated by my spiritual body".
Its like learning to gain conscious control of your body heat with a sensor toy, it gives you tangible feedback to focus on while learning.
Without it, you could say...accidentally transplant your liver function into the air instead of your spiritual self. This would be very unwise.