It can be a turning point in the story. If the fanfiction was previously focused on Sakura's independent actions, this immobilization can change the direction completely. She might have to rely on others or use her wits in a different way to get out of this situation. Itachi's actions here can also introduce new plot elements, such as secrets he reveals while holding her or new alliances that form as a result of this encounter.
It could also be related to protecting her. In the Naruto universe, there are many powerful and dangerous forces. Itachi might see that Sakura is in over her head in a situation and by immobilizing her, he's ensuring her safety until he can figure out how to deal with the threat. For example, if she's about to engage with a foe far stronger than her, he steps in to stop her.
His possessiveness could be seen in his actions towards Sakura. He may be overly protective, like not allowing her to go on dangerous missions alone. He would always want to be the one to accompany her, ensuring her safety at all times. This shows that he doesn't want to share her with any potential threats that could come her way during a mission.
It might signify a shift in their relationship dynamic. Maybe the writer is exploring a new, unexpected connection between them.
One possibility is that Sakura tries to communicate with Itachi. She might try to reason with him, asking him why he has her captive. This could lead to some revelations about Itachi's true intentions. If Itachi is part of some bigger plot, Sakura could find out crucial information during this conversation, which could either make her situation worse or give her a chance to escape.
Itachi and Shisui both being Uchiha have a deep bond based on their clan heritage. When Sakura enters the picture, she might initially feel like an outsider. But as they work together, Sakura's intelligence and combat skills earn her respect from the two Uchiha. Itachi might be initially cold towards Sakura but gradually come to respect her, while Shisui is more open - minded and quickly befriends her. They could form a triangle of mutual respect and support where they each bring different skills to the table.
She might be a bit confused at first. After all, Itachi has a very complex and somewhat dark past. But she could also grow to understand her father's perspective as she learns more about Itachi herself.
Sakura's character could develop by becoming more confident. Training with Itachi, who is a highly skilled ninja, would expose her to new levels of combat and strategy. This would make her trust in her own abilities more. For example, she might start taking more risks in battles after learning from Itachi.
In this fanfiction, Itachi might help Sakura become stronger by training her in various ninja skills. He could teach her advanced genjutsu techniques. For example, he might start with the basics of detecting and breaking genjutsu, and then gradually move on to creating complex illusions herself. This would enhance Sakura's ability to defend against and use genjutsu in battles.
Sakura's character could develop in terms of her understanding of the ninja world. Itachi and Shisui have different outlooks compared to her. Through training with them, she might gain new perspectives on things like loyalty, sacrifice, and the greater good in the ninja world, which in turn shapes her character.
Sakura's amusement could actually make Shisui more likely to continue annoying Itachi. Since she finds it funny, Shisui might see it as a sign that his actions are acceptable and entertaining. This could potentially strain the relationship between Shisui and Itachi further if Itachi doesn't like being the target of Shisui's annoyance.