In order to "break" a format, you have to find a deck that is far more powerful than anything else that anyone could be doing or, more commonly, find something that attacks on a different angle that is also very powerful.
Did Ivan Jen break the format? It's hard to say. We'll basically just have to wait and see if his deck continues to perform well. There's not really any arguing that it's doing something similar than everything else in the format.
At its core, it's a simple Heroic deck -- use some cheap spells that target your creatures and cantrip, which should find you more things to target your guys. Conveniently, this plays well with prowess as well. Jeskai Ascendancy is the big payoff here though. With Ascendancy in play, you'll have access to a lot more card selection and will be able to pump your entire team. If you have some Springleaf Drums, you can use those to generate mana. If you have Retraction Helix, you can bounce your opponent's board.
The biggest concern I have with the deck is the manabase. With all the cantrips in the deck, having such a low land count doesn't surprise me (especially considering that Springleaf Drum is essentially a land), but nine "enters-the-battlefield-tapped" lands is a lot -- it's half of the manabase!
Overall, the deck seems very powerful, although it might have some consistency issues. Since it won the tournament, I wouldn't be surprised if people pick up this deck and start practicing with it. Over the next couple of weeks, we should have a pretty good idea of how good it is.
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