In the underground training arena:
Boom.
With a whistling gust, Shinji threw a punch, effortlessly toppling the wooden training dummy in front of him with a low, muffled thud.
"Something's off…"
Shaking his head, Shinji shifted two steps to the right and threw another punch at a new dummy.
Zooming out, the underground arena was littered with fallen wooden dummies, scattered haphazardly across the floor.
Boom.
Another low thud echoed as yet another dummy fell!
But Shinji's face showed no trace of satisfaction—his brows were tightly furrowed.
"Is it an issue with the breathing rhythm?"
"Or is the chakra stimulating the heart and lungs too early?"
"Or perhaps the muscle contractions are insufficient?"
Mulling over these questions, Shinji approached another dummy.
These dummies were crafted at great expense, made from ironwood sourced from the harsh, northern lands of the Land of Lightning and the Land of Iron.
Their hardness rivaled stone, yet their flexibility surpassed it, making them the ideal material for taijutsu training dummies.
Staring at the dummy before him, Shinji didn't rush to strike this time. Instead, he reflected, "According to Ryūhei Ubuyashiki, breathing is merely the surface. The true essence of the Breathing Technique lies in the coordination and resonance of the entire body—a method to unify all organs as one. My narrow focus on stimulating the heart and lungs seems to have backfired…"
Shinji began to grasp the truth: the core of the Breathing Technique was coordination, with breathing merely a means to achieve harmony across the body's organs.
With this realization, his thoughts expanded.
"Unlike the swordsmen, I'm a shinobi with chakra. I can use chakra to directly stimulate the body's organs without being bound by 'breathing' alone…"
The Demon Slayer Corps swordsmen, lacking chakra, had no choice but to rely on breathing to engage their heart and lungs, gradually achieving bodily coordination. This required relentless, day-after-day training and exceptional talent and willpower to succeed.
Shinji was different.
With chakra, he could directly stimulate the necessary organs with precision and ease!
This alone put him leagues ahead of every Demon Slayer Corps swordsman!
"I never thought the first step in optimizing the Breathing Technique would be to strip away everything related to 'breathing,'" Shinji muttered, finding the irony amusing.
With a clear direction in mind, he left the training arena and returned to his study. He spread out all the Breathing Techniques, including Sun Breathing, on his desk and began the process of fusion and elimination!
Between the various Breathing Techniques, there was no clear hierarchy of strength.
Sun Breathing was the orthodox root of all Breathing Techniques, but that didn't mean the derivative techniques lacked merit.
Shinji categorized dozens of Breathing Techniques based on their focus, then matched them to specific organs, extracting their strengths and discarding their weaknesses to refine and condense them.
For example, among all the Breathing Techniques, Sun Breathing had the greatest effect on training the heart, so he adopted the heart-training aspects of Sun Breathing.
Similarly, Water Breathing was the most effective for training the kidneys, so he incorporated its kidney-training components.
Matching the heart, brain, liver, spleen, lungs, kidneys, stomach, intestines, and other organs one by one, Shinji drew from the strengths of each technique, condensing dozens of Breathing Techniques into a single, unified one.
Since it directly targeted the body's organs, the condensed Breathing Technique wasn't bloated—on the contrary, it was clear and structured.
Its only drawback was that, unlike the original techniques, it lacked a distinct focus. In other words, the condensed Breathing Technique sacrificed the sharpness of pushing a single aspect to its extreme, becoming more restrained and comprehensive.
The old Breathing Techniques could push a practitioner to the pinnacle in a specific area, while the condensed version offered stability and all-around enhancement.
"It's got a bit of the 'Sword vs. Aura' debate vibe," Shinji mused, weighing the pros and cons.
The old Breathing Techniques were like the Sword Sect—focused and quick to yield results with great power. The condensed version, however, resembled the Aura Sect, emphasizing deep accumulation with limited power before mastery and greater difficulty in training.
In the Demon Slayer world, Shinji's condensed Breathing Technique would be utter garbage.
Even a prodigy like Yoriichi Tsugikuni couldn't have trained in it, let alone an ordinary swordsman.
But for a shinobi with chakra, though the training difficulty remained high, chakra served as a "cheat code." With dedication, most could achieve results!
After completing the theoretical fusion, Shinji returned to the training arena.
Paper plans were static. To ensure the dozens of Breathing Techniques could be seamlessly integrated without interference, practical testing was necessary.
Standing before another dummy, Shinji applied his condensed Breathing Technique and threw a punch with force.
Boom.
Amid the explosive sound, the dummy didn't just fall—it was obliterated, wood chips scattering across the floor.
Yet Shinji's face flushed red. Staggering a few steps, he coughed up a mouthful of blood.
It took a long while for him to recover. Wiping the blood from his lips, he muttered, "Damn it, I didn't expect the first attempt to injure my lungs and organs. Matters involving the body's organs really do affect the whole system!"
That single punch, due to its exertion method, had damaged his left lung and part of his liver, causing internal injuries.
His condensed Breathing Technique consisted of twelve forms, each with at least five different exertion methods. If he tested them one by one like this, he'd likely die before perfecting the technique.
"No wonder history's great creations, beyond masterful skill, always required a bit of luck," Shinji chuckled, then thought, "I may not have the luck to be favored by fate, but I have eyes that can peer into it!"
With that, he activated his Eternal Mangekyo Sharingan, his inky black eyes turning crimson!
The exertion methods of the Breathing Technique produced immediate results, so Shinji could use Izanami no Mikoto's fate-peering ability to test them through fate's branches, avoiding reckless attempts that could harm his body.
Soon, his vision filled with images of himself throwing punches.
Some punches were weak and powerless, some caused him to collapse immediately, others led to him spitting blood endlessly…
As he peered into these visions of fate, a smile crept onto Shinji's lips…