Having already left, Kōki knew nothing of Yūhi Shinku's complicated thoughts. All he wanted now was to sort through what he had gained today. The theories on Yin Release that Yūhi Shinku had discussed had given him a great deal of inspiration. His mind was now full of ideas he wanted to test.
Kōki kept thinking about Yin Release the entire way home, reaching his front door before he knew it. He took his key out of his pocket and was about to unlock the door when he noticed several footprints outside.
Realizing something, he looked at the leaf wedged in the door crack and knew that whoever had come looking for him had not entered. After opening the door, he checked his windows. The hairs caught in the window gaps were still there too. Only then did he breathe a sigh of relief, certain that no stranger had broken into his home.
He still had a few things at home that were best kept out of sight, such as some Naruto plot details he had written in Chinese characters. Although no one in the Shinobi World could recognize those characters, it was still better to keep them hidden if he could.
Still, he wondered just who would come looking for him.
But he soon put that question out of his mind. Whoever had come looking for him had not entered his home without permission, which meant they at least bore no ill intent.
What mattered more now was finding paper and a pen immediately and writing down what Yūhi Shinku had taught him today.
With that thought, Kōki immediately fetched paper and a pen from his room and wrote down everything he had seen and learned that day. As he wrote, inspiration flashed through his mind, and he suddenly thought of another ninjutsu that involved Yin Release: the most common ninjutsu, the Shadow Clone Technique.
He hurriedly fetched a scroll from his room. It contained instructions for practicing the Shadow Clone Technique, but more importantly, it did not merely record the technique. It also included the original records of its development and research, the fruits of Tobirama Senju's painstaking efforts.
Hiruzen Sarutobi had given it to him. When handing over the scroll, Hiruzen Sarutobi had looked very serious, saying that it was extremely precious and that he hoped Kōki could gain something different from it.
Kōki had taken out the scroll now to read the research notes at the back. He remembered skimming through the entire scroll when he first received it. After the training instructions came the development records for the Shadow Clone Technique, which included a few mentions of Yin Release.
At the time, however, Kōki had focused on the training instructions at the beginning and had only glanced at the rest.
Now that he remembered the scroll, he immediately took it out to compare it with the notes Yūhi Shinku had given him. Kōki opened the scroll, skipped the training instructions at the beginning, and went straight to the research notes at the back.
The records on the scroll stated that the Shadow Clone Technique had originally been developed as a cannon-fodder ninjutsu, a technique intended to reduce ninja deaths during intelligence-gathering operations.
At first, the Second Hokage Tobirama Senju had tried to develop a simplified physical Clone Technique based on his elder brother Hashirama Senju's Wood Clone, but he found that he could not develop it no matter what he tried.
After repeated failed experiments, he discovered that his brother's Wood Clone contained a considerable amount of Yang Release chakra. He therefore wondered whether an excess of Yang Release chakra was preventing the physical Clone Technique from succeeding. He then reduced the input of Yang Release chakra, but he still could not create a physical Clone Technique.
Just as he was at a complete loss, he suddenly remembered a legend. According to it, the Sage of Six Paths had used Yin-Yang Release to create the world, as well as the ninjutsu that ninja used today. If so, did that mean the union of yin and yang was the true essence of ninjutsu?
With that question and excitement, Tobirama Senju began experimenting with the Clone Technique again. This time, however, he divided the chakra of the two attributes, yin and yang, into equal halves and tried using the Clone Technique.
Sure enough, the clone that appeared this time was no longer an illusion and had a physical body. Strangely, however, its expression was dull, and its movements required conscious control from the original. It did not meet his desire for a physical clone with a consciousness of its own.
At first, he suspected that the clone's low intelligence might be because he had not supplied spiritual energy at the same time as the two chakra attributes.
So he tried releasing spiritual energy together with the yin- and yang-natured chakra. This time, the clone was noticeably different. There was already a trace of awareness in its eyes, but it still could not act on its own.
After yet another failed experiment, Tobirama Senju felt a hint of frustration. He could clearly sense that this physical clone technique was only one step away from completion, but he could never find what was missing.
Unwilling to give up, he began poring over the vast collection of documents held within the Senju Clan. The earliest of these dated back to the era of the Sage of Six Paths.
After spending a great deal of time searching the Senju Clan's collection, he found a single statement about Yin Release: Yin Release used Yin Power, which drew upon the spiritual energy governing imagination, to create form from nothing.
That single statement struck his mind like a bolt from the blue. Wasn't giving the physical Clone Technique he wanted to create a consciousness of its own precisely a matter of turning the formless into something with form?
Feeling that he had found the key, Tobirama Senju conducted another experiment, redistributing the proportions of yin and yang chakra. With Yin Release chakra as the primary component, Yang Release chakra as a supplement, and spiritual energy supporting them, he finally succeeded in creating a physical Clone Technique. Because these clones possessed the same consciousness as the caster, he named it the Shadow Clone Technique.
The Shadow Clone Technique he developed still had a certain flaw, however: when a shadow clone was struck and disappeared, the pain and memories of that blow would also return to the caster's mind.
This made Tobirama Senju realize that although he now had shadow clones, they still could not serve as the suicide squad he wanted. After making a great many fine adjustments, Tobirama Senju finally produced the Shadow Clone Technique that spread throughout the Shinobi World.
After reading Tobirama Senju's development records, Kōki could not help admiring his approach to research: bold hypotheses, careful verification.
Tobirama Senju had seen the possibility of a physical Clone Technique in his elder brother's Wood Clone and boldly hypothesized that he could use Yang Release to develop one. After repeated failed experiments, he had drawn another hypothesis about the origins of techniques from an elusive legend. Then, through careful verification, step by step, he had finally succeeded in developing the Shadow Clone Technique. He truly lived up to his reputation as Naruto's research expert and master of forbidden techniques.
After marveling at how badass Tobirama Senju was, Kōki took out Yūhi Shinku's research notes and began studying and comparing the differences between the two ninja's approaches to Yin Release chakra.
He hoped to find the ultimate method for Yin Release that he wanted in these two sets of research notes today.
Just like that, Kōki became absorbed in an ocean of knowledge, rapidly drawing the essence from the two sets of Yin Release research notes and making it his own.
Kōki felt his control over Yin Release chakra growing increasingly proficient and his understanding of Yin Release deepening. Suddenly, a spark of insight flared in his mind. He felt that he, too, might be able to use Yin Release chakra to perform a special kind of Clone Technique.