When the final step is completed, a Rasengan can be formed, which compresses high-density Chakra into a ball that spins rapidly.
This step is not easy, because to concentrate high-density Chakra into a ball, you must maintain high-speed rotation of the Chakra in the palm and also sustain a stable input and the speed and direction of Chakra's rotation.
Under such circumstances, unless you have the talent for multitasking or have a ninja familiar through years of practice, it is hard to form such a powerful technique as the Rasengan.
In the original story, to solve this problem, Naruto had to resort to clever methods.
He summoned two Shadow Clones, with the main body maintaining the stable input of Chakra, while the clones sustained the high-speed rotation and speed and direction of Chakra's rotation.
