Haki Shintai

Haki Shintai is an advanced technique of physical combat accessible only to high-level practitioners of. According to Kurama, it literally means 'the mind and body coming together through Haki'. As such, it is a skill only those who have mastered the individual heights of both and  can ever hope to achieve. While no creator of the technique has ever been attributed, it is said to be a level of combat that distances regular warriors from the truly powerful, and therefore, it is said to be the natural conclusion every Haki master eventually stumbles upon during their training.

Overview
The human body is modeled in a very specific and distinct manner. Every signal, every impulse, every reaction; these are all natural aspects of the human body and compose the instinct that determines action. Martial art training can allow a person to cultivate their instinct and reactions to better suit their combat style, and through repetition and battles, this is even further developed. Put another way, training and experience allows a person to better control their own impulses in order to use their body more efficiently in combat. However, there is a limit to what one can do with mere physical training. No matter how much one trains, it is impossible to physically change certain signals or impulses put out by the brain, especially in regards to certain muscles or internal organs. It is not possible for a human to give orders to each and every one of their muscles down to the individual nerves.

However, through mastery of Kenbunshoku and Busoshoku Haki, one can manage to learn to detect the flow of Haki within their own bodies, and then, enhance the flow while controlling its path. With enough precision, it eventually becomes possible for a Haki master to quite literally overtake all of the nerves in their body with Haki, to such a degree that every signal and impulse is replaced with the user's Haki. In managing to do so, the user attains complete control over every facet of their body, down to the smallest of nerves. Every single muscle becomes theirs to control.

Attaining such absolute command of the muscles enables the user to move in rather inhuman ways. In other words, the user is able to control their body in a way that nature did not intend for a human body. One such example of this is being able to move all of one's muscles simultaneously and instantaneously on a whim, allowing one to accelerate their movements to full speed without requiring build up. This instant 0-to-100 acceleration causes most movements by the user to resemble blurs, as if they were constantly teleporting short distances. Furthermore, Haki Shintai causes a drastic increase in offense and defensive capabilities, now that the body's muscles are all functioning as one rather than being devoted to individual functions.

To those who lack experience with Haki Shintai, however, the usage of it can be dangerous. Having full control of the body means also having to account for minor functions such as heartbeat and blood flow. These functions are normally handled by the body naturally, but assuming full manual control means needing to focus on maintaining those functions while fighting. Otherwise, the user can drop dead the moment they try to initiate the technique. That being said, this is considered a double-edged sword, for full autonomy also means being able to keep certain functions active even if they would normally shut down. For example, should the heart come under attack by a contagion or poison, the body would respond by shutting down. But with absolute control of the body, the user can manually keep their heart beating even under such conditions until managing to weather through the abnormality.

Behind the Scenes
This technique was inspired by the swordsmanship of "Twin-Wings" Edelweiss from the light novel series, Rakudai Kishi no Cavalry.