"Come on, let's do it," said Jiraiya eagerly, holding a Rasengan in his palm. "Let's have them clash together and see what happens."
Experimentation is one of the most important steps of the scientific method!
"Are you mad?" Sakura blurted out. "Do you know just how much chakra I've got spinning in my hand!?"
The Rasenshuriken was more than merely two Kakashis' worth of chakra condensed into a sphere scarcely larger than a cricket ball. With every heartbeat Sakura sustained the technique, that chakra edged ever keener, the sheer forces swirling within coaxing it into countless, microscopic needles— each so fine they could permanently sever chakra pathways.
Damage on so minute a scale is simply irreparable— Chakra pathways are far too intricate for even someone with Tsunade's legendary touch to operate on. That being the case, Sakura was hardly eager to have her Jutsu detonate in her own face.
"The technique isn't terribly useful, truth be told. It's far too dangerous for the user," Sakura went on. "The only remotely safe delivery method I've found is using Shadow Clones, and even they're hardly what you'd call reliable."
"Oi," said the pink-haired Shadow Clone resentfully.
It wasn't for want of trying that Sakura had abandoned the notion of hurling the Rasenshuriken.
The moment the sphere left her hand, the roiling chakra trapped beneath its containment layer would almost instantly burst free and detonate prematurely. Unless Sakura were somehow one with nature itself, it was impossible for her to compel it to hold its shape once it got too far away from her.
"Observe," Sakura told the spectators. "Why don't you go hit that boulder over there?" she then instructed her Shadow Clone, placing the Rasenshuriken in her awaiting hands. "There's a good girl."
Now, Jiraiya and the others understood why Sakura had summoned a Shadow Clone earlier!
"So, that's what the Shadow Clone was for," remarked Kakashi.
Naturally, Sakura's doppelganger was hardly keen on carrying out the task that she had been created for (especially since it would end very painfully for her), but orders were orders all the same. Swallowing a few choice curses under her breath, she seized the Rasenshuriken and set off at a brisk pace towards one of the few boulders in the Third Training Ground that Sasuke had not yet reduced to rubble.
"Wind Style: Rasenshuriken!"
Sakura was instantly hit with the feedback of her Shadow Clone's pain.
"GYAAHHH!"
The moment the Rasenshuriken struck the boulder, it unleashed the razor-sharp wind chakra sealed within, utterly disintegrating the unfortunate doppelganger. A huge ball of pure white light, generated by the friction of countless chakra strands grinding together, flared outwards, lingering for a full ten seconds. When the howling winds finally subsided and the light show abated, nothing remained of the boulder but a fine pile of sand.
"Whoa…"
Naruto, Sasuke and Kakashi stared at the devastation Sakura had wrought in shock.
"You know, when Naruto taught you that Jutsu, I never imagined that you would be the one to take it to the next level." said Jiraiya in amazement. "So, what do you call it?"
"The Rasen…" Sakura began, then caught herself just in time. "Er…"
"The Rasener?" asked Naruto, tilting his head in confusion.
If she became known as the Rasenshuriken's creator, then wouldn't Tenten find it really suspicious if she picked the exact same name Naruto had given it? She had to find something else…
"The Rasenchakram!" Sakura blurted out. "Because of the disc of air that forms around the ball's equator!"
It was the name Sakura had dreamed up when first struck by the notion to create a Rasengan shaped like a sawblade. Sadly, inspiration had struck her rather too soon, before her understanding of chakra had fully matured. The concept proved unsound, so she had quietly shelved the name and left it to gather dust… until now.
"I thought it looked a bit like a shuriken, with the spinny bits sticking out," suggested Naruto. "Why not Rasenshuriken, y'know?"
"Because the Jutsu can't be thrown, idiot," scoffed Sasuke.
Sakura cleared her throat discreetly.
Technically speaking, the Jutsu's namesake, an Indian weapon shaped like a flat disc known as the chakram— could be thrown as well as wielded in close combat. Hang on… that name might actually be quite fitting after all! After all, she'd be able to throw it, one day…
All in all, Jiraiya was thoroughly satisfied with the demonstration.
His pupil Minato had drawn inspiration from the myriad Tailed Beast Balls flung about during the Third Shinobi World War to invent the Rasengan. The whole point of coming up with that Jutsu was so that his beloved Kushina might wield the signature attack of the Tailed Beasts, without needing the Fox's cooperation. To Jiraiya, it felt most fitting for Minato's son's girlfriend to refine that original Jutsu for his son's sake!
Jiraiya made a mental note to ask Tsunade to have Sakura transcribe the training method, so that he might have a crack at it in his own free time later.
"Well?" Sakura asked in anticipation. "Will you teach me a Jutsu, now?"
"A deal's a deal," Jiraiya declared, crossing his arms and nodding firmly to himself. "A real man never goes back on his promises, and I am a man among men!"
For the System to deduce the ultimate technique that transformed men into women, Sakura needed to assemble a long shopping list of requirements, including but not limited to…
"That technique that turns other people into frogs," said Sakura. "That's the one I want to learn. It sounds rather fun."
Jiraiya almost did a double-take. Why on earth would this girl ask for a prank technique he had developed in his boredom during his youth? It couldn't exactly be used in combat, so it was virtually useless except for intimidation and interrogation purposes!
"And yes, I'm sure," said Sakura stubbornly, cutting off Kakashi before he could even try to counsel her into making a wiser choice. "That's the one I want."
"It's your funeral, girl," said Jiraiya, shrugging.
…
The Kaeru Kaeru no Jutsu's name is a pun on the fact the words 'frog,' and 'change,' are homonyms… as expected from a technique a thirteen-year-old had dreamed up after visiting Mt Myōboku once.
It can only be used in either of the following three scenarios:
The caster's chakra and proficiency in chakra control are vastly superior to the victim's.
The victim's chakra is sealed.
The victim is attacked by surprise before they can mould any chakra.
Essentially, the technique works a lot like a Genjutsu, in that the caster sends their chakra through their victim's pathways and makes their chakra flow in a certain way. Basically, it forces the victim to perform a Transformation Jutsu and change into a useless frog.
For the uninitiated, the technique can be really quite terrifying, but because of the massive size difference between humans and frogs, it sadly can't be held for too long.