The Reincarnation Technique can indeed strengthen the soul, but its principle isn't based on Chakra for soul reinforcement.
Instead, it somewhat utilizes Natural Energy. To cultivate the Reincarnation Technique, one must first meditate and contemplate, reaching a state of 'sitting and forgetting' where there is no joy or sorrow, no thoughts, and no self. (Author's personal setting, please don't criticize if you dislike it.)
However, most people generally can't achieve a state of no distracting thoughts, let alone the inherently evil Senju Tobirama.
Therefore, he drew inspiration from some of the Shinobi Sect's 'unity of heart and heaven' and the Buddhist Sect's 'Zen meditation' methods to develop a special incense, somewhere between a sachet and burning incense.
Using these incenses, named concentration incense, one could easily enter the state of 'sitting and forgetting,' bringing their soul vibrations closer to Natural Energy.
A portion of Natural Energy would passively permeate their soul, diluting it and slowly absorbing it, thereby strengthening their soul.
Then, when the soul is strengthened to a certain stage, it can exit the body and float several meters away in a fully enclosed room.
Further strengthening allows it to move further and further from the body, eventually enabling nocturnal excursions, which we call 'night wandering.'
Finally, when it can travel during the day, it reaches the limit of the Reincarnation Technique, allowing the soul to enter and control another person's body.
I have to say, if it weren't for the different worldviews, Senju Tobirama would have surely developed the path of Yang God and Ghost Immortals.
However, what angered Ling the most was that the concentration incense had already been used up by the freeloading Kato Dan, and because a certain species went extinct at the end of the Sengoku period, later generations couldn't cultivate this technique.
This is also why Orochimaru could so easily give the Reincarnation Technique to Ling; he hadn't mastered it either, and even the concentration incense couldn't be researched for a substitute due to the lack of actual samples.
Of course, you can also forcibly enter the state of 'sitting and forgetting' on your own and then cultivate the Reincarnation Technique.
Unfortunately, in the Ninja world, with constant fighting and killing, where is the time to eliminate distracting thoughts and achieve self-forgetfulness?
By the time you spend many years to enter this state, others who practice Ninjutsu would have long since become strong enough to easily defeat you.
The state of 'sitting and forgetting' is even higher than the state of mind-emptiness and unity with heaven required for cultivating Sage Mode, which is why no one has successfully cultivated it.
Ling naturally finds it difficult to cultivate now, so it's a useless skill, much like the inherently evil Orochimaru.
Feeling irritable, Ling fell into a sleep, beating Orochimaru soundly in her dream, venting her frustration.
In the following days, Ling diligently practiced various nature transformations and the Gale Sword Technique.
One day, as Ling went out for her morning run to train her physique, she saw a cloud of dust spreading towards her from a distance.
At the forefront were two green-clad bowl-cut figures, moving at an incredibly fast speed.
Ling only had time to cover her mouth and nose before being engulfed by the dust, only breathing fresh air after a long while.
"Damn bowl-cuts, my good mood this morning is ruined."
Ling recognized these two as Might Duy, the Genin who single-handedly defeated the Seven Ninja Swordsmen of the Mist, and Might Guy, who almost kicked Six Paths Madara to death.
Before Ling could walk away, the two bowl-cuts ran back and stopped in front of Ling.
"Yosh! Sorry, little girl, the dust got on you." Although Might Duy was sincerely apologizing, he gave a thumbs-up, and the flash, brighter than a black person's teeth, directly blinded Ling.
"Um ~ Uncle, what brand of toothpaste do you use?"
Ling directly asked a question that troubled many Hokage fans, but it silenced Might Duy and his son.
Logically, shouldn't this girl have cursed them like everyone else? Why is she asking about toothpaste?
"Yosh! ~ Little girl, we don't use toothpaste; we use homemade tooth powder. Isn't the effect great?"
Might Duy once again gave a thumbs-up with a tooth flash, and Ling felt that this move could be used as a Ninjutsu, directly blinding enemies.
Didn't Six Paths Madara have a Ninjutsu that made his teeth glow? It was called Gale Style: Light Fang.
"Oh, okay, but why don't you train at the public training grounds?"
Ling felt that the two father and son running like this on the street was also a problem.
"Yosh! ~ This ~ This is because our training makes too much noise and often disturbs other Ninja, so ~ ~"
Might Duy was a bit embarrassed, as being a perpetual Genin, he was often driven away by younger Ninja, which was truly difficult to mention.
"Oh ~ I see. Then, how about you train with me? Anyway, there aren't many people at my training ground."
Of course, there weren't many people. That was the Uchiha training ground, and no one dared to challenge the Uchiha.
"Eh ~ Can we? Yosh! ~ Little girl, you're truly a good person. I'm Might Duy, a Konoha Genin."
"Cool! I'm Might Guy, a Konoha Genin."
Might Guy graduated in the same class as Obito. Originally, Guy, who hadn't learned the Three Body Technique, wouldn't have had a chance to graduate.
However, with the Third Shinobi World War about to begin, the number of Ninja was somewhat insufficient, so naturally, someone lowered the graduation requirements.
"I'm Uchiha Ling; you can call me Ling."
Ling could only give a slight smile in response to the father and son's friendly thumbs-up and tooth flashbangs.
"Uchiha?" Might Duy clearly knew the reputation of the Uchiha Clan, so he was a bit surprised at this moment.
Ling immediately understood what Might Duy wanted to express, "Uncle Duy, I'm a half-blood. My father is Irming Yagyu; you can also call me Ling Yagyu."
"Oh, Yosh! ~ Thank you, Ling. Let's rush to the training ground together, Guy!"
"Hai! ~ Father, is this youth? Oh ~ I finally understand!"
Might Guy and his father embraced, tears streaming down their faces, looking extremely excited.
A cold sweat unconsciously trickled down Ling's forehead, feeling a strange awkwardness at this moment.
[You have gained two new stable emotion providers. Currently, there are three stable emotion providers.]
Ling was a bit happy, not expecting Might Duy and Might Guy to be so easily fooled. She hadn't even exerted herself, and they had already surrendered.
Directly gaining two stable emotion providers so easily, now she only needed seven more people to draw a prize again, which was truly not easy.
Arriving at the training ground, Shisui had already begun his Shuriken throwing practice.
"Ling, you're here? And these two are?" Shisui didn't recognize the Might father and son, although their bowl-cuts were very recognizable.
"Shisui, this is Uncle Duy and Might Guy. I know you want to form bonds with the Village Ninja, so let's start with the Might father and son. They are both good people, and their Taijutsu combat power is super strong."
Ling directly found a somewhat unreasonable reason for her actions, and Shisui indeed always talked about forming bonds with the Village Ninja to change the relationship between the Village and the Uchiha Clan.
However, Ling saw that Shisui only talked about it every day but never made good friends with any Ninja in the Village, so Ling decided to help.
"Thank you, Ling." Shisui truly wanted to do that, but he had never made friends with others, so facing the first step, people would walk away as soon as they saw he was from the Uchiha Clan.