Harry might use his wits and the environment around him while in captivity. For example, if he's held in a place with magical artifacts, he could use them to his advantage. He could break the chains that bind him with a spell - enhanced object and then make his way to Voldemort and defeat him.
In this fanfiction, it could be that Voldemort has a premonition. He sees that if Harry Potter dies, it will lead to a series of disasters that will also destroy his own plans. So, in order to preserve the balance or to keep his long - term goals intact, he saves Harry. For example, if Harry's death causes an uncontrollable chaos in the wizarding world, Voldemort's rule over the dark forces might be disrupted. So he steps in to save Harry.
It could start with a lot of animosity on Harry's part, but as he is trained, he might start to see Voldemort in a different light. Maybe he begins to understand Voldemort's motives, though not necessarily agreeing with them.
Well, he might use his knowledge of magic in unexpected ways. For example, he could use a simple spell in a complex situation that Voldemort wouldn't anticipate. Harry has always been good at thinking on his feet.
Harry in this fanfiction is likely to be more ruthless. Voldemort's upbringing would make him less empathetic. He may value power above all else and be more willing to use dark magic to achieve his goals. His relationships would also be different. He wouldn't have the same close - knit group of friends as in the original story. Instead, he might be surrounded by Death Eaters or those who follow Voldemort's ideology.
Harry could become more powerful in a way that scares his friends. Voldemort's training might focus on the Dark Arts, which are powerful but dangerous. This could change Harry's magic style. He may become more solitary as his new skills and knowledge set him apart from his Hogwarts friends who follow a more traditional path of magic.
Voldemort might initially be furious. He's not used to being challenged in such a blatant way. He could try to destroy the Howler immediately in a fit of rage.
Well, in some fanfictions, Harry might use some hidden magic he discovers in Azkaban. Maybe he finds a way to turn the Dementors' power against Voldemort. Since Dementors can suck out a person's soul, Harry could somehow direct them towards Voldemort, weakening him enough to deliver the final blow.
It changes the canon by introducing a new form of interaction between Voldemort and Harry. Instead of just the typical battles and mind - reading attempts we see in the original books, the 'plugging' concept adds a more invasive and direct connection. For example, Voldemort could directly influence Harry's thoughts and actions in ways that were not possible in the canon.
Harry would likely be defiant at first. He has a strong sense of justice and would not easily submit to Voldemort's punishments. For example, if Voldemort uses the Cruciatus Curse, Harry would probably try to resist the pain and not show weakness.