Haruto worked diligently with Hiruzen, converting the spiritually charged chakra coating in his throat into balanced chakra coating bit by bit.
It took some time and a few close calls with "OOohs!" and "Aaahhs!" but Haurot managed to come out relatively unharmed on the other end.
Hiruzen looked at the sky that still had a little light left in it and asked Haruto, "We still have a little bit of time to learn one more topic. So what do you want to go with first? Explosive power or speed?"
Haruto frowned in thought but only for a second as Megumin immediately shouted inside his head, 'Explosion!!!!'
'Fine…' Haruto responded with a mental nod as he declared his intentions to Hiruzen, "I'd like to learn more about bettering my explosive power today Kakashi-san."
Hiruzen looked at the childish gleam in Haruto's eyes and chuckled slightly before nodding, "As good a choice as any. Both the topics use similar principles but in slightly different ways. Let's start with the explosion side of things however, since that is what you picked!"
Haruto's eyes focused as he did his best to absorb the information being presented to him by a master.
"To improve the explosion of your fireball you need to use one of the inherent properties of chakra that you might've learnt about while practicing the leaf concentration exercise… can you reason out what that is?"
Haruto frowned, 'While practicing the leaf concentration exercise?'
Focusing on the memories he had received from his clone, Haruto scanned through them before stumbling upon the right answer.
"The explosion of chakra caused by coalescing a lot of chakra in a single tenketsu? But then how does that apply to a jutsu, don't jutsu have no tenketsu? Otherwise does simply overloading a fireball with more chakra make it more explosive?"
Hiruzen smiled at Haruto's initiative before speaking slowly and explaining things one at a time.
"You are right, we will be using the property of explosion of chakra that is caused by stuffing a tenketsu with a lot of chakra. But that does not mean we will simply store the excessive chakra inside the fireball itself, that would be a waste of chakra and take too much of it to improve our explosion."
"Is there a better way?" Haruto asked with slight interest.
"Yes, and that is by creating a tenketsu within the fireball jutsu itself."
"Creating a tenketsu…" Haruto wanted to question how that was possible until he remembered the Shadow Clone jutsu he had learnt courtesy the Dark God.
'Ah… he heart tenketsu can copy any tenketsu chakra passes through before it reaches that into the jutsu right?'
But Haruto didn't say it out loud.
Nevertheless, Hiruzen noticed his question and answered it.
"The tenketsu in the heart is slightly special, it can copy all the tenketsu chakra passes through before entering it into the jutsu. In our case, since we already pass the chakra through the tenketsu in the solar plexus, the heart tenketsu will copy it and itself into the jutsu. Try it out, why don't you, that'll make everything clearer."
Haruto then closed his eyes and focused on his chakra circulation system before creating some more fire nature chakra and the passing it through the heart tenketsu. It took him about ten seconds to do this. Almost instantly like when he created a shadow clone, Haruto felt an internal connection to the jutsu.
'That must be the tenketsu formed inside the jutsu.'
Haruto exploited this connection and filled it with even more fire release chakra before expelling the entire thing out of his throat.
This time however, Haruto didn't aim at Hiruzen and instead aimed at the empty ground slightly far away.
Turned out to be the right call.
"BBBBOOOOOOOOOOOOOOOOOOOOOOMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM!!!!!!!!!!!!!!!!!!!"
Haruto felt himself lifted off his feet and thrown backwards as a thirty meter tall mushroom cloud rose up in the distance.
Hiruzen immediately moved behind Haruto and caught him mid-flight before helping him brace for the impact by shielding him.
Several shards of glass and rock flew towards Haruto and Hiruzen but Hiruzen blocked it all with his body in the heat of the moment. Yet because his transformation jutsu didn't falter, Haruto remained oblivious of the numerous cuts and scratches that battered Hiruzen's body.
The cuts weren't anything too harmful however since they were shallow enough to heal within seconds given the ridiculous healing rate of shinobi in this world but Hiruzen was definitely going to be needing a change of clothes.
A prideful look showed in Hiruzen's eyes as he looked at Haruto, "Good job, Haruto-kun. That was a fantastic explosion."
Haruto opened his eyes fully as the explosion subsided and finally looked at the damage done.
There was a five to six meter crater built as a part of the training ground now. The sand nearby had been turned into glass because of the heat and the rest of the sand looked charred, completely burnt of any organic content.
Haruto's gaze then shifted towards Hiruzen's pride filled gaze before he too felt slightly proud about his own new jutsu.
But that was only until Hiruzen spoke, "That was a decent attempt for a beginner. While your jutsu itself is pretty good, your execution is not. Given your lack of chakra control and your lack of usage of handsigns your jutsu execution takes you more time than you can afford on the battlefield."
Haruto's eyes widened and he agreed, "Makes sense…"
"And that is apart from the fact that simply using that one jutsu, puts you out of all the chakra you had in you…" Hiruzen explained as Haruto was panting slightly with sweat forming on his forehead.
Hiruzen then looked up at the sky once more and seeing the last of the light dip below the horizon, he quickly spoke again, "Let's end today's session here. You may practice the jutsu for the next week, that's when we'll have another class. Is that understood?"
"Understood sensei!!"
Hearing Haruto call him sensei, Hiruzen's chest filled with some pride but then it quickly deflated after he thought of the trouble this would bring him if word got out that he'd taken up a new student.
'Bah! I'll face it when the time comes. Until then…'
He looked at Haruto and smiled, "Good job today. I'll see you next week then."
Haruto returned the smile and spoke quickly, "Thank you sensei!"
And with that Hiruzen was gone. Not even a swirl of leaves was left behind as Hiruzen flickered away from his spot.
Haruto looked at the spot Hiruzen had flickered away from a little wistfully before smiling and shaking his head and making his way back home.
Once home, Haruto didn't simply eat his ramen and sleep, instead he quietly pulled out a fresh new notebook from the shelves before writing down his experiences in training under Hiruzen today.
Along with that he wrote down several more questions that he would focus on during this one week training period he had too, like, 'What happens if more chakra is poured into the heart tenketsu?'
'What happens if the tenketsu in the jutsu is not saturated with chakra , will the fireball travel further in that case since there is no unstable element in it?'
And even 'Does speed increase by increasing the expelling speed from the tenketsu? Perhaps it increases by filling the throat tenketsu with too much chakra to cause an explosive release?' and 'Does increasing speed then penalise the size of the fireball since some of the chakra is wasted in the explosive release?'
As Haruto finished with those he realised he also had other more practical questions.
He'd felt his throat begin to heat up in spite of having coated it with chakra, did that mean that you couldn't continuously fire out fireballs?
In the end, after noting down all the questions Haruto carefully stowed away the small notebook and decided he would experiment with all these in the upcoming week.