The ability to cast a powerful burn spell for little or no mana was a huge draw and led to some potent double-spell turns early enough in the game that the opponent couldn't recover. With Mantis Rider now making a new home in Modern, it's no surprise to see its old friend Stoke the Flames come along for the ride.
Goblin Rabblemaster has been replaced by the cheaper and more versatile Young Pyromancer, which still gives you the ability to double- or triple-spell very early in the game by recouping the mana spent on cantrips or other removal spells via convoke. With other powerful burn spells like Lightning Bolt and Lightning Helix, this deck is great at closing the door once an early lead is established, even in a format with as much cheap disruption as Modern.
The commitment to a more aggressive plan is clear from the inclusion of Spell Pierce, a card that isn't good to draw in the mid-game but is great at pushing an early tempo advantage. I'm sure it catches a lot of players off-guard because most of this deck looks like typical Jeskai. It's also a nice card to have in combo matchups since you'd like to tap out on the early turns as much as possible, leaving a window for them to race.
The sideboard lets you transition into a more typical midrange or control role via powerful planeswalkers and a couple of copies of the now-ubiquitous Logic Knot to have a catch-all when the game goes long. That transitional sideboard plan is straight out of the Jeskai decks of Khans of Tarkir-era Standard, though this one isn't as pronounced, since you still want to have enough aggression to turn the corner quickly in Modern.
I'm a big fan of the Molten Rains in the sideboard here as the best answer to big mana decks. With so much burn in the deck already, the two points of damage likely will be relevant, and the one-turn setback is all you need to close the door. When you play a longer, reactive game, you need to shut down your opponent, but once you go the aggressive route, just forcing a stumble is sufficient. The ease of forcing that stumble is part of the power of being proactive.
Jeskai isn't just for control mages anymore.