Well, in some fanfictions, it could be during a life - or - death battle. Maybe when he's facing an extremely powerful enemy that he can't defeat with his known abilities. His friends could be in great danger, and that intense situation triggers the revelation of his hidden power.
In some fanfictions, Naruto's hidden power could be the ability to control a powerful and ancient force. For example, he might be able to harness the power of some long - forgotten bijuu or a mysterious energy that was sealed away in the past. This would give him abilities that are on a completely different level compared to his normal skills.
He could be using basic chakra control techniques to suppress his power. By constantly regulating the flow of his chakra, he can keep his true power hidden. For example, he might be using a form of internal chakra suppression similar to what some ninja use to hide their presence.
It also depicts his growth in terms of his chakra control. In fanfiction, Naruto could gain the ability to split his chakra into multiple clones without any limit. These clones would not be the normal shadow clones but more like independent entities with their own thoughts and powers, which he can control simultaneously. This would make him an extremely formidable opponent. Another way is that his chakra becomes so pure that it can heal any injury, not just physical but also mental and spiritual ones. This new aspect of his power would change the dynamics of his battles and relationships with other characters.
Well, often in Naruto fanfic, Naruto can get power from the Kyuubi when he manages to break through the mental barriers that were set in the original series. He might be able to communicate better with the Kyuubi and convince it to lend him more power. It could also be that some external force or event in the fanfic world allows him to access the Kyuubi's power in a new way, like a magical artifact or a new type of jutsu.
In certain fanfic concepts, Naruto could get the phoenix power during a near - death experience. His strong will to survive and protect his friends could attract the phoenix spirit. The phoenix, known for its rebirth and powerful abilities, might choose Naruto as its host, seeing his indomitable spirit as a worthy vessel for its power.
Well, it could be that Naruto's strong will and his connection to all the other tailed beasts play a role. Since he has befriended the other tailed beasts, their combined energy or some sort of spiritual link might attract the power of the Juubi. For example, in some fanfic concepts, the tailed beasts' collective consciousness decides to merge their powers with Naruto, which includes the Juubi's power as the ultimate form of chakra.
Naruto's character could become more confident. With the Sage of Six Paths power, he knows he has the ability to protect those he loves and the entire ninja world. He might not second - guess himself as much as before when facing challenges.
In fanfiction, Naruto can power up in various ways. One common way is through new jutsu training. For example, he might develop a super - powerful Rasengan variant that combines different elements. Another way could be by unlocking hidden bloodline limits. Sometimes, fanfiction writers give Naruto the ability to tap into some ancient or forgotten power source within himself.
Another possibility is that Naruto has his own plans and goals that he wants to achieve independently. His true power might be a key factor in these plans, and he doesn't want his parents' influence or interference. He wants to prove himself on his own terms, without relying on his family connection.
In this fanfiction, Naruto's power development could also be related to his perception of the world. Aizen's own view of power and the world is very different from what Naruto has been exposed to. Through their interactions, Naruto might start to see things from a different angle. This change in perspective could lead to a growth in his power as he discovers new ways to use his abilities. For instance, he could start to understand the weaknesses in his opponents more clearly and use his powers in a more targeted and effective manner.