It looks to me like the Horcruxing of Harry is likely what gave him much of his power, with that power lending credibility to his “defeat” of Voldemort, and avoiding suspicion of Harrymort when he displays so much power after the upload.
...What power? Wasn’t it explicitly called out that Harry’s dark side had no superpowers and he wasn’t any stronger than any other highly-motivated first year?
I like that. It does seem like Harry’s dark side is the one that can find a win in any situation, and that does seem to be his strongest power—just the will to win, and the means to calculate that win.
But there is also:
For once, just once, Harry hadn’t gotten shortchanged in the mysterious powers department.
After almost a month of work, and more on a whim than any real hunch, Harry had decided to make himself coldly angry and then try the book’s Occlumency exercises again. At that point he’d mostly given up hope on that sort of thing, but it had still seemed worth a quick try -
He’d run through all the book’s hardest exercises in two hours, and the next day he’d gone and told Professor Quirrell he was ready.
His dark side, it had turned out, was very, very good at pretending to be other people.
...What power? Wasn’t it explicitly called out that Harry’s dark side had no superpowers and he wasn’t any stronger than any other highly-motivated first year?
But Harry has The Ultimate Power.
I like that. It does seem like Harry’s dark side is the one that can find a win in any situation, and that does seem to be his strongest power—just the will to win, and the means to calculate that win.
But there is also: