"Care to elaborate how everyone else in Konoha failed to do it?"
"First of all, users need to have affinity to lightning, which is the first problem where the majority of shinobi in the land of fire have fire affinity." Kakashi pointed out where lightning jutsu was the rarest scroll found in Konoha, until 3rd Hokage took ownership of Uchiha library.
"Second, you need to have mastery over lightning chakra, which has separate training, much harder than the basic one, which takes years to finish, which is not possible for anyone to do, due to mission you will be delegated by village and the information of this training is restricted to few clans which withhold information for their own interest." Kakashi said with a bitter smile.
He recalled that the 3rd Hokage was called professor because he was obsessed with mastering 5 elements to the point, he avoided participating in the 2nd and 3rd Shinobi wars.
'No idea why he was obsessed with it, he hardly fought except in self defense and it's not like he was busy inventing a new jutsu.'
'Also, I don't understand why he never revealed the training information to Anbu as well...'
"Kakashi sensei?"
"Ah right *cough*."
"Third, you need mastery over chakra transformation, or chakra control in simpler words, which was difficult to determine until the 4th Hokage invented a jutsu that helps you to learn mastery of it." Kakashi asserted, where Sasuke understood he was talking about Rasengan, the pinnacle of chakra transformation.
'I guess I learned Rasengan early...'
He recalled the scene where it was called an incomplete jutsu created by 4th Hokage, where he did not manage to infuse the element.
Then he realises one thing:"Hey, wait a minute, why is learning your clan kenjutsu requirement the same as creating a new jutsu? "
Kakashi pulled out reforged white light chakra sabre and began to demonstrate and instructed Sasuke to activate his Sharingan.
Kakashi then infused chakra into the sword:"If you see here, due to competent chakra control, one can fuse their chakra into the sword."
"However, only at the surface, which is already exposed to the surroundings. Its core hardly gets infused and it is impractical to do so as it usually causes the fragility of the sword to increase."
Sasuke don't know why he need to open his "2 tomoe Sharingan" as only Bakyugan can able to see colour and shape of chakra that well, until Sasuke noticed that Kakashi's Tantō, glowed light blue, where it had chakra flow and chakra colour, akin to Kakashi had.
"Unlike normal infusion, if you had mastery over chakra and chakra control, you can instead extend and manifest your chakra from your body to the sword, linking sword to the body's chakra, hence becoming one of the 'limb' of the user."
"The main advantage, if you have your sword made of chakra metal, is that there would be no wastage of chakra, compared to chakra infusion."
"Wait, how much of an advantage is it?" Sasuke asked, not clear how different it was from normal fusion.
Kakashi sighed and replied:"If you infuse your chokutō with chakra, chakra is consume to maintain it, where consumption rate is increases to maintain when you attack the weapon with your chokutō, as chakra that was infused will be lost when get contact to enemy body, object or jutsu and rate of increase will based on how much chakra you infused."
"However, if you extend and manifest your chakra into your chokutō instead, damage it will cause without further addition or chakra, will be equivalent to highest chakra infusion user could do and chakra manifested will be not be lost, where it will stay intact, which is useful especially at long fight where you can't able to use any more jutsu." Kakashi concluded, which surprised Sasuke.
"And if you were wandering, no, it's not financially possible to make a battalion out of it even if you have lightning affinity users, where even if you gave chakra metal sword to use and sword made of quartz to train, one's lightning affinity must be stronger than the average, one's body must be strong enough to endure the gruesome training to get mastery over chakra control and lightning chakra and ones must have mastery of swordsmanship to use it properly."
"Then why are you bothering to say this is it impossible to do?" Sasuke questioned, feeling there was a reason why he was saying this to him.
"Well, you have good training ethic, good at copying jutsu thanks to your Sharingan, natural at kenjutsu and have lightning affinity, so lets say, it's the second last lesson I am teaching you, as I doubt you would need to learn from me afterwards." Kakashi said, believing it was natural for him to pass the torch early to him and his teammate after the Chūnin exam as they are more than capable to protect Konoha.
They were geniuses academy did not notice.
And most importantly they adhere to his core belief, to have your comrades back regardless of the situation.
"So, what is the last lesson you will teach me?" Sasuke asked, which caused Kakashi to grin.
'Suppose I sneak peak won't be that bad..'
Kakashi then pulled out his right arm.
First it was glowing light blue, where its vibration emitted was akin to hearing one thousand birds chirping and sparks from it were going haywire.
Sasuke covered up his ears, due to proximity to it and put some distance when few sparks hit him.
Then the sound of it decreases and sparks emitted decreases, allowing Sasuke to let go of his hands.
At last, it turned purple, where not only sound was absent, there were no sparks coming from it.
Looking at appearance, Sasuke knew what Kakashi 'perfected' jutsu he was talking about.
'Purple lightning.'
Kakashi then retracted his arm and canceled his jutsu, seeing his demonstration was enough to impress Sasuke.
"It's a jutsu I managed to create after sealing my Sharigan, as chakra consumption is much higher, but in exchange I have absolute control over it."
Kakashi had a genuines smile, feeling relieved that he does not rely on his Sharingan to perform his strongest attack and won't make mistakes he made.
Such as Rin's death.
"Now let's start our training." Kakashi said with a smile, wiping his tears before Sasuke noticed it.
Sasuke, nod looking forward to learning that move, especially for doing stronger variations of Chidori current, great for defense at close range.