Jeskai Ascendancy was supposed to break Modern. With Treasure Cruise it seemed destined to, but it didn't stand up to the various fair decks that could utilize the legitimately broken card draw spell with plenty of cheap interaction. After Treasure Cruise was banned the deck hasn't had the requisite power to become a consistent player in the format.
But this isn't the traditional list. You won't find an ugly green splash for mana creatures to both accelerate into Jeskai Ascendancy and generate mana while comboing. This list relies solely on Fatestitcher and a lone Faerie Conclave to execute the combo, supplementing it with a more controlling gameplan with six maindeck removal spells, the maximum four Remands, and Snapcaster Mage.
The card that unites these two disparate plans: Monastery Mentor. Even if you don't win immediately, making an army of Monk tokens should set up an easy kill on the following turn against most decks, and you don't even really need Jeskai Ascendancy for that to work.
Turns 1-3 - Answer their stuff.
Turn 5 - Cast a boatload more cantrips and attack for a lot of damage.
Both pieces are powerful alone, but they get even sillier together. I can easily imagine games ending with you attacking with an army of thirty Monks, all of which are over 10/10 in size. There's something a little unsettling flavor-wise about Monks being so aggressive, but we're playing to win here.
Playing a longer game helps you extract the most possible value from Visions of Beyond, which is one of the major payoffs of the archetype. Flashing it back with Snapcaster Mage for three more cards is such a huge resource edge that you'll be able to piece together a win in myriad different ways.
That longer game also lets you make a lot of land drops, which you do because you have a ridiculous twenty cantrips, so don't be fooled by the low land count. Once you hit four, five, or even six lands, you can set up a single turn combo with Fatestitcher, thereby not exposing your Jeskai Ascendancy as often. Your opponents will be eager to land more threats as you kill their first few, opening the window for that combo turn as well.