Yuichi smiled and scratched his head, a bit embarrassed by the commotion he had caused. "My apologies, I had an emergency that I need to attend to back home. Everything's fine, and Mei is fine too. She should be in class #1 right now with Kira. I'll explain later. Let's not make a scene here. Everybody's looking at us. You guys better take your seats before Instructor Kensei arrives."

“Oh right you were gone so you wouldn’t know. Instructor Kensei said he had a mission to handle so Instructor Rin took over so he’s currently teaching both the science and combat lessons.” said Hanatarō

“Although he never seems excited about the combat lessons. He’s always complaining that this isn’t his area of expertise and another person would be better. I agree with him but I assume it was hard to get another instructor on such short notice.” added Niko

Yuichi acknowledged Niko and Hanatarō's information with a nod of thanks. As Instructor Rin arrived and the class settled into their seats, there was an air of anticipation and curiosity among the students. The room fell silent as the instructor began to speak.

"Good morning, everyone," Instructor Rin addressed the class, his voice steady and dispassionate. "Today, we won't be engaging in a combat lesson. Instead, we will focus on strengthening our bodies using your reiryoku. I’ll show you a demonstration and then you’ll all attempt it. Please follow my instructions closely, now let's begin."

Yuichi's interest was piqued, as he wondered if the way he had created would match up with how soul reapers enhanced their bodies with their reiryoku. His method was rather like those of the Quinices in the sense he not only saturated his muscles but also mixed it with his blood like they did when they used arterie.

As Instructor Rin proceeded to explain and demonstrate the reiryoku-based body-strengthening techniques, the classroom was filled with a sense of anticipation. Yuichi, always eager to learn and improve his skills, listened intently and observed the instructor's demonstration.

Instructor Rin's method appeared to involve a unique blend of reiryoku manipulation and physical enhancement. As the students watched, he channeled his spiritual energy into his body, causing his muscles to expand and harden, enhancing his physical strength and agility. The demonstration was impressive, showcasing the potential for reiryoku to be harnessed in a way that went beyond traditional combat techniques.

Instructor Rin continued his demonstration and then proceeded to explain the theoretical basis behind the reiryoku-based body strengthening techniques to the class.

"Listen carefully, everyone," he began. "The method I just demonstrated is based on a fundamental principle of our existence as soul reapers. Our bodies are composed entirely of reishi, the spiritual particles that make up the world. Reiryoku is the condensed form of reishi, and it's what allows us to perform various abilities, including our zanpakuto techniques."

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He walked around the classroom as he explained, engaging his students. "To strengthen a specific part of your body using reiryoku, you need to understand that the process is akin to recreating the conversion of reishi into reiryoku. However, this is difficult since we do so unconsciously. In other words, you must be able to consciously change the reishi that composes a particular body part into something akin to reiryoku, albeit temporarily."

The students listened with rapt attention, and Instructor Rin continued, "By doing this, you effectively transform the reishi within that body part into a more powerful energy source, enabling you to draw on enhanced strength, speed, and resilience. This method is about harnessing the reishi within yourself and utilizing it to improve your physical attributes. It's a practice that, when mastered, can significantly amplify your combat capabilities. Note that all captains have mastered this ability and are able to use it unconsciously when they attack and defend granting them explosive power in those moments"

Yuichi was intrigued by this explanation. He thought about the implications of manipulating the composition of reishi within his own body to enhance his abilities, similar to the way he converted reishi into reiryoku. Due to having reincarnated here it never occurred to him that he was a being composed entirely of reishi. The theoretical understanding provided by Instructor Rin opened up new possibilities for incorporating this knowledge into his training and combat.

Yuichi posited that Arrancars must do this at all times. An arrancar technique that Yuichi never understood was Hierro (鋼皮 (イエロ), "Steel Skin") which granted them steel-hard skin strong enough to block a zanpakuto with their bare hands. He realized that this might be a continuous process for Arrancars, who instinctively condensed their body's reishi into reiryoku, resulting in their superior physical abilities compared to even captain-class soul reapers.

However, soul reapers still defeated them, meaning Rin’s method was most akin to an explosive burst in strength as opposed to the arrancar’s constant method. In other words, a captain class soul reaper might be a 5 out of 10 while an arrancar would be 8 out of 10, but using this method would temporally allow a captain to reach 10 out of 10.

This method allowed soul reapers to temporarily reach the pinnacle of their physical capabilities. This insight led Yuichi to see the potential of combining these different approaches. Soul reapers could tap into the power hidden within their own bodies, unleashing it when needed to achieve remarkable feats of strength and speed, while maintaining their versatility and spiritual pressure control. So if he incorporated this into his unique training method, and slowly started to get a feel of transforming his body's reishi into reiryoku, he could develop the physical capabilities of an arrancar.

Yuichi observed as the other students in his class attempted to replicate Instructor Rin's demonstration. Many of them struggled to grasp the concept at first, but as the exercise continued, one by one, they began to show signs of progress. Yuichi remained patient, allowing his fellow students the opportunity to figure it out on their own while he thought more about his theory.

Yuichi faked many failures as he waited. It wasn't until more than half the class had managed to complete the assignment that Yuichi decided to make his move. In his usual manner, he quickly followed each step, demonstrating a keen understanding of the technique. With the newfound insights he had gained earlier, the process felt almost more natural to him than it seemed for the others. He focused on manipulating the composition of reishi within his arm into reiryoku.

Within moments, his arm began to show signs of enhancement, a faint but discernible shift in its density and power. Yuichi successfully executed the technique, showcasing the potential benefits of understanding his own reishi-based bodies. Instructor Rin came by felt Yuichi’s arm and said “Pass,” as he did with all the other students before.

The rest of the lesson was uneventful as Yuichi and the others who succeeded would repeat the process over and over, as to get a feel for it until class ended. When class ended Instructor Rin left them with these words. “Remember this ability will be your lifeline in the future. We won’t revisit this subject again, so it’s up to you to refine it. Simple things like this will determine your future prospects as to whether you shall one day rise up the ranks.”