What is the basis of Ninjutsu? It wasn't the seals. It wasn't knowing the technique. It wasn't familiarizing oneself with it. Not even chakra control, although the latter helped a lot.
The basis of Ninjutsu was the movement of chakra.
The seals were a guide to mold it in the correct and efficient way. But what truly made that understanding possible were the scrolls.
Tetsumaru and Mei theorized, after a few years of study, practice, and error, that those scrolls not only contained the name of the technique and the seals to activate it.
They must have also had a diagram with the Tenketsu points through which the chakra passed and the correct sequence to execute the Jutsu.
They reached that conclusion because their pure chakra manipulation was simply too good. Had they been reincarnated into the Naruto world, they would have almost never had to use seals for techniques.
With such manipulation, it was impossible that they hadn't already developed several Ninjutsu on their own, despite not knowing the seals for all of them.
What stopped them then? They could only assume, and later confirm, that it was the lack of information about the chakra pathways.
They were not sensors. They did not have the Byakugan. They couldn't know, or remember, where the hell each and every one of those points was. Of course, they could move their chakra almost at will through their body, but it had to be in a more or less large quantity or they wouldn't feel where it was.
So they could only roughly estimate where those damned points were. But they couldn't test indiscriminately.
There was a reason why almost no one created techniques every day. It was very, very dangerous.
Once, Mei had tried to exhale fire. She believed that by having the Lava and Steam Release Kekkei Genkai, her affinity with Fire would be high enough to be able to execute that Elemental Release with her pure manipulation.
The result? She didn't burn her throat, or her stomach, or anything like that. By moving her chakra carelessly, she "injured" one of those chakra points and ended up with an irritating pain in her chest.
She knew the seals for the fireball perfectly, and her chakra control was high quality. There was no other explanation for such a failure than the movement of chakra.
From that day on, they stopped trying to replicate famous techniques with pure internal chakra manipulation.
Over the years, they refined their control to such a point that they dared to try again, but with more care and nothing that would hurt them too much, while also being a practical and useful technique.
That is how they arrived at the last two years of developing the Shunshin. For two years, they studied the enhancement of their bodies through chakra with great care.
For two years, they more or less determined the areas where the chakra should flow for that movement technique.
They discovered that it wasn't just about the legs, although they did have to concentrate the largest amount there.
They needed to reinforce their hips, back, neck, and arms so as not to break anything with the abrupt movement.
After overcoming that stage, they had to find the correct route through which their chakra should flow along their points, which was where they were now.
It was Tetsumaru's turn to move. Through their bond and in absolute silence from Ragdoll's perspective, Mei transmitted her previous experience to him, so the shinobi slightly altered the movement of his chakra.
He shot off, moving at a speed that once again stunned the PussyCat, but did not achieve the desired result for either him or Mei.
—D-damn it!— he hissed through their bond, making a grimace that he hid from the heroine. —Let's not try combination P-7, 10, 13, and 19— he warned his other half, communicating the dull pain that made his toes rigid.
Those numbers indicated the estimated location of the pathways in their legs. They had already made a sketch of all the points they more or less knew in their bodies and had memorized it.
It was for safety reasons, to avoid this type of situation.
Both halves continued their exhausting practice for another couple of hours, which Ragdoll observed in absolute silence.
The hyperactive heroine got a little bored but did not interrupt once. With her Quirk, she could notice some things, but there was a lot she couldn't see and she didn't know what they were trying to achieve.
When she asked them, neither was willing to share it with her. As such, she couldn't offer advice.
She assumed they were trying to increase their movement speed, but if she couldn't even understand how their internal energy worked, there was little she could contribute to their training.
It wasn't until well into the night that they stopped. Although they tried to hide it, Ragdoll realized that these practices caused them pain.
Slight trembling in their legs, sweat on their foreheads, and grimaces they forced themselves not to show. The heroine took note of everything to report it to their Sensei.
The next morning, the students' day of torture continued as normal.
Mei once again gave a harsh lesson to the thick-headed students who just threw themselves forward with everything, without showing true combat skills.
She also practiced with Ojiro, Kendo, and Shoji, giving them advice and helping them refine their reactions when facing a faster enemy.
For his part, Tetsumaru and Pixie Bob subjected Todoroki and Bakugo to an intense session of creating ice and explosions to avoid being swallowed up in a sea of earth and iron sand.
Endeavor's son was somewhat impressed by Tetsumaru's ability to mold geometric bodies that crushed and pierced his ice.
For Sero, Tetsumaru created a small sand jungle with pillars and spikes in all directions, forcing the boy to cross the middle, swinging like Spider-Man without touching the ground once.
Mei used TetsuTetsu, Kirishima, Sato, and Iida in a cruel endurance competition, making the four run through a furious torrent of water.
From the side, Kōta couldn't help but observe the kunoichi at all times, feeling a sea of emotions when looking at a water user.
For their part, Aizawa and Kan monitored the condition of both siblings, vigilant in case of any injury, given what the Pussycat had shared about their training.
"You have to talk to them," Kan commented to Aizawa with a serious look. "It's not right that your students don't even trust you, their teacher, to help them with their problems."
"It's hard to trust someone who doesn't trust you," Aizawa replied dryly and with a hint of regret.
He was not unaware of the siblings' attitude. For some unknown reason, they had always been cautious about discussing their abilities, either with their classmates or with the school itself.
Aizawa didn't doubt that both were aware of the suspicions against them. They knew that Ragdoll surely evaluated them and reported to Aizawa. They knew that he sent the girl with them to watch them, not to look after them.
The only thing he didn't know was whether this reservation was due to their intrinsic nature or if there was something more.
He wondered if they also realized that there might be a spy in UA. He wondered if, in case they weren't the spies themselves, that possibility was what made them so distant.
But Kan was right. A student who didn't trust their teacher and a teacher who didn't trust their student was a relationship from which nothing good would come.