Naruto Uzumaki’s signature jutsu may have been the Rasengan, but it was never truly his own. The Rasengan was the highlight of his arsenal in the original Naruto, but later in Naruto Shippuden he upgraded the technique and created the Rasenshuriken. In doing so, he had finally created something that was uniquely his.
Witnessing Sasuke’s growth firsthand reminded Naruto that he wasn’t yet at the level he wished to be power-wise. The immature challenge he’d thrown at Sasuke back when they were younger may have been the last thing on his mind, but he needed to get stronger if he was going to rescue his friend. His new objective seemed simple enough, but it was easier said than done.
In making the technique his own, Naruto, luckily, had Kakashi Hatake on his side. His sensei decided the fastest way to accumulate strength and experience would be to try creating a powerful, original jutsu. Normally, this process is long and arduous. For example, creating the Rasengan took the Fourth Hokage three years and it was still an incomplete jutsu. Naruto didn’t have the luxury of time though. Fortunately, Kakashi's experience allowed him to hatch a work-around for his pupil.
After helping Naruto figure out his chakra nature, Kakashi had him train to master the wind nature. He instructed him to use as many shadow clones as he could create simultaneously. This would drastically reduce the training time needed since Naruto retained all the experience accumulated by each clone when he released the jutsu. Kakashi's genius training method let Naruto clear several months of work each day. It did carry the risk of enduring a proportional amount of fatigue, but that wasn’t enough to stop him.
As Asuma Sensei explained, the wind nature was perfect for cutting. Naruto’s training regimen included trying to cut through various objects using his chakra alone: first through leaves, and then an entire waterfall. It wasn’t an easy task but after some help from Asuma, Yamato and Sakura’s deadly-tasting food pills, Naruto mastered the wind change in chakra nature.
His new jutsu was far from finished though. In order to upgrade the Rasengan, Naruto had to figure out how to add his newly mastered nature transformation to it. Incidentally, this was a feat both Minato and Kakashi had failed to achieve. Minato succeeded in creating the peak change in chakra form but was unable to realize his true goal of adding his chakra nature to it. Kakashi likewise attempted to complete his sensei’s work but even he couldn't manage it.
In aiming to add a nature transformation to the Rasengan, Naruto had his work cut out for him. His initial efforts yielded no results and he declared the task as impossible as trying to look left while looking right. It was an offhand comment, but after seeing Kakashi do exactly that with a shadow clone of his own, Naruto got an idea.
He applied the same trick he’d used while learning the Rasengan. This time, he attempted the jutsu with two shadow clones. He held the forming jutsu himself while one clone focused on making the Rasengan, then the second clone added his wind nature. Instead of a dissipating Rasengan -- the result of all his previous attempts -- the Rasenshuriken now spiraled into life.
Naruto may not have known how powerful his new jutsu was just yet, but that didn’t make the Rasenshuriken any less deadly. When hitting a target, the Rasenshuriken would propel it backward with great speed and power just like the parent Rasengan, but it didn’t end there. After transporting its captive a significant distance, the Rasenshuriken explodes into a giant sphere filled with millions of microscopic needles.
At this stage, the Rasenshuriken’s most sinister effect goes into play. The needles are small enough to pierce through the skin with no apparent damage -- in actuality they attack the target’s internal organs, severing every connection to their internal chakra network. Even if they manage to survive the Rasenshuriken, the opponent loses all ability to mold or use chakra. Tsunade classified it as a jutsu forbidden to even Naruto for a while. In his hands, just creating them was damaging to the chakra network.
The Rasenshuriken quickly became one of Naruto’s signature attacks. He debuted it against Kakuzu, where a single hit was enough to destroy two of the Akatsuki members' hearts. Naruto also defeated several of Pain’s paths with this jutsu, but its most iconic use was perhaps against Sasuke during their final battle. There, Naruto created two massive Rasenshurikens, one using a massive amount of Tailed Beast chakra and natural energy, the other being an actual Tailed Beast Bomb.
