What Is a Seal?

At the training ground.

Sakumo sat on a dirt mound while Katsuki stood before him, head slightly lowered as he listened attentively to Sakumo explain his understanding of ninjutsu.

"Ninjutsu, Taijutsu, and genjutsu are all rooted in chakra. They are different methods of application developed according to each person's talents. You can't say which is stronger, only which suits you better. Some people excel at ninjutsu, some prefer genjutsu, and others train Taijutsu diligently. It varies from person to person. For example, I'm a ninjutsu-type ninja."

Sakumo raised his right index finger, where bright arcs of lightning danced with faint crackling sounds. Then he raised his middle finger as well, which turned iron-gray as dim light flowed over it.

"Sensei, then what type am I?" Katsuki asked softly, forcing himself not to look at that middle finger.

"I don't know."

"Huh?"

Sakumo laughed in irritation and glared at Katsuki.

"How could I possibly tell what type you are right now? This classification is only something I came up with myself. Whether you excel at ninjutsu, genjutsu, or Taijutsu is something you have to explore on your own. Why ask me? How would I know?"

He gave Katsuki a rapid-fire scolding.

Katsuki sheepishly scratched his head and chuckled foolishly. He had been thinking about it the wrong way. Right, if something like this could be seen at a glance, wouldn't that be absurd? Still, he could not help wondering which of the three—ninjutsu, Taijutsu, or genjutsu—he had the greatest talent for.

"There is one more thing you must remember. Any top-tier expert may master one path, but must never focus exclusively on one. Take genjutsu, for example. You may not be skilled at casting genjutsu, but you must at least learn how to dispel it. Otherwise, you'll be at a serious disadvantage against enemies who use genjutsu."

"Of the three arts, Taijutsu is considered the lowest because its offensive power cannot compare to ninjutsu, and it is not as elusive and difficult to deal with as genjutsu. But it is also the foundation of all foundations. A ninja's chakra is limited, and ninjutsu cannot be cast without restraint. Taijutsu remains the most commonly used. That is why I require you to temper your body and practice swordsmanship. If you can kill with Taijutsu, use Taijutsu whenever possible."

Katsuki listened in rapt fascination. Neither the teachers at the Ninja Academy nor the elders of the Uzumaki Clan had ever analyzed the differences between ninjutsu, Taijutsu, and genjutsu so clearly. Though he possessed a great deal of memories from his previous life, they had only skimmed the surface. He had never held such a profound understanding.

"Genjutsu is not my specialty. What I can teach you are Taijutsu and ninjutsu." Sakumo coughed lightly. "You can barely be said to have entered the door in Taijutsu. Your blade work is still raw, but once you kill a few more people, you'll naturally become proficient."

Hearing that, Katsuki bared his teeth in a silent smile. His teacher truly was ruthless to the core. He spoke of killing people for practice as casually as slaughtering chickens.

"Earth Release: Rock Rods."

Sakumo clapped his palms together, and a stone rod suddenly shot up from beside Katsuki's feet. Though Katsuki reacted quickly enough to leap backward, the rod still grazed his cheek, its protruding stone edge tearing open a bloody gash.

Katsuki was startled. Touching the blood on his face, he felt like crying without tears. What was this supposed to be?

"How did that feel?" Sakumo asked with a smile.

"..." Katsuki angrily wanted to complain, but when he was about to speak, he saw Sakumo's gentle smile and swallowed the words back down. Sakumo did not like putting on empty displays; every movement he made had a purpose. The scene from just now replayed in Katsuki's mind... and he suddenly cried out, "No hand seals? Th-this isn't scientific!"

Katsuki stared in astonishment.

"Sensei, how did you do that? Could it be a no-seal ninjutsu?" Katsuki came up with what seemed like a reasonable explanation.

"No-seal ninjutsu... You really know how to imagine things!" Sakumo laughed.

His hands began moving, forming one seal after another. He deliberately slowed down, and then... another stone rod sprang up. This time, Katsuki was prepared and managed to avoid getting injured, but his eyes had already gone wide.

There was no doubt that it was the same ninjutsu. With hand seals and without hand seals, it had been cast both ways.

This nearly overturned Katsuki's ingrained understanding of the ninja world. It was not no-seal ninjutsu, yet it could be cast without seals. He could find no explanation for it. He could only stare wide-eyed at Sakumo, waiting for a reasonable answer.

"Katsuki, what is the process for casting ninjutsu?"

"Form hand seals, refine chakra, then use ninjutsu."

Katsuki answered without hesitation.

"Correct. Then let me ask you this: why do we need hand seals to use ninjutsu? Why can't we simply refine and use chakra directly?" Sakumo fixed his bright gaze on Katsuki.

Katsuki fell silent. His eyes widened, and he had no idea how to answer.

"Let me put it another way. What do you think a seal is?"

This time, Katsuki lowered his head and thought seriously. What was a seal? This question somehow felt even more fucked up than the previous question of "what is ninjutsu?" Was there any use in knowing this? Wasn't it enough to simply form seals faster?

His mind was blank, but Katsuki still forced himself to think. A flash of inspiration passed through his mind, and after hesitating, he said, "A seal should be something like a mechanism for controlling chakra, right?"

Sakumo did not answer immediately. He merely smiled, his eyes full of approval.

"A seal is control—control over chakra transformation. When we cast ninjutsu, we use hand seals to correspond with the chakra within our bodies, allowing it to transform smoothly before we use it. That is the entire process of casting ninjutsu," Sakumo said, enunciating every word.

"As for why I can cast ninjutsu without seals... care to guess?" Sakumo looked at his disciple with a smile.

Katsuki was instantly a mess. Who said honest people could not joke? He cut him off at the crucial point. Th-this... this friendship could not continue.

"All right, I won't tease you anymore." Seeing Katsuki's twisted expression, Sakumo burst out laughing and gave him the answer. "Nothing more than practice making perfect."

Practice made perfect?

Holy shit, what a powerful explanation—so powerful that it was hard to believe.

"What? You don't believe me?"

Sakumo raised an eyebrow. He was not surprised by his young disciple's reaction, because most ninjas believed firmly in the process of forming seals before casting ninjutsu. Such notions could be called deeply ingrained.

"Seals exist to control chakra transformation. But once you have cast a ninjutsu hundreds or thousands of times, you become thoroughly familiar with the chakra transformation process. At that point, you can naturally cast the ninjutsu without forming seals."

Katsuki was speechless. He made it sound so simple.

"Of course, being thoroughly familiar with it does not mean you should abandon hand seals. Casting without seals does have drawbacks. First, you cannot achieve fine control over chakra. In other words, you waste a little chakra. For experts with limited chakra reserves, that is intolerable."

"Second, the power of the ninjutsu will be weakened to a certain extent. After all, without precise control of chakra, the ninjutsu naturally cannot unleash its proper might."

"Third, you dare not abandon hand seals for Fire Release techniques, because flames are too destructive. If they get out of control, even a single spark drifting in the wrong direction could lead to uncontrollable consequences."