The sideboard in this deck is incredibly important. It is in other decks as well, but not to this extent. Siding in certain cards in certain matchups can swing the odds in your favour by quite a lot. Lets take a look at the cards that make the Magic happen!
The Top End
These cards are the ones that allow us to survive the late game.
Chandra, Pyromaster's +1 ability is absolutely amazing in this deck. Being able to ping our opponent for 1 and make a creature unable to block, we can win out of nowhere. Her 0 ability is pure card advantage. Like Abbot of Keral Keep's ability, it will rarely be wasted. It will either be used as "flood insurance" or card draw. Chances are we will have the mana to cast whatever she exiles. Her ultimate will rarely be used, but it would be fun to finish someone off from 12 life by exiling a Boros Charm
Narset Transcendent is another great late game card. Her +1 is a lot of the time just going to say draw a card, because of all the instants and sorceries we run. It will at least give us an idea of what is coming up. Her -1 is where she really shines. Being able to rebound a burn spell or anything else really is great, especially with all the Prowess triggers we get for free. Once again her ultimate is rarely used, but could be an alternate win condition.
Monastery Mentor is possibly the best Prowess creature out there, and an amazing finisher in this deck. Once he sticks he can take over the game by himself. With his tokens and our spells combined, he has an insane board presence. He only isn't in the mainboard because game one we want to be as fast as possible, and he takes a little bit of time to take over the game.
Sword of Fire and Ice is a unique top end card, but fits the theme perfectly. It triggers Prowess it buffs a creature, and creates pseudo unblock-ability and of course protection. Along with all these things, it does extra damage, which can also act as removal, AND is a draw engine. This card truly does it all and is great for the top end of this deck.
Grindy Matchups
These are the cards that come in for the grindier matchups such as midrange and control.
Dispel comes in to win counter wars and to protect our important threats when we need them to stick.
Jeskai Ascendancy is a great card to provide additional card advantage and extra explosiveness among all the deck's creatures. The only issue lies in the fact that it takes a few turns to build an advantage. That being said, in game 1, we need to be as fast as possible, but in game 2 and 3, we need the extra advantage against slower, more controlling decks.
Specific Matchups
These are the cards that come in to deal with specific cards in certain matchups.
Deflecting Palm comes in against any burn or combo deck. It throws the damage right back at them.
Wear / Tear is quick artifact removal for affinity, and more importantly, quick enchantment removal against Blood Moon decks. A great, versatile spell for several matchups.
Vapor Snag is a great removal spell that has the added bonus of doing a damage. It is great because there are things that our burn spells just can't deal with. Especially with Eldrazi decks running around the format now, we need an answer. The only time it isn't good is when it enables the opponent to abuse ETB triggers. That being said, we can use it on our own creatures as well, whether to protect them, or abuse ETB triggers.
Celestial Purge offers another great answer to several decks. It takes care of Grixis Control, deals with big eldrazi creatures in the R/B variant, and also deals with Blood Moon.
Spellskite is possibly one of the most flexible sideboard cards ever. It comes in against burn, auras, or anything with alot of single target spells.
Engineered Explosives is a great board wipe. It comes in against zoo, tokens, or any deck that runs alot of permanents with the same CMC.
Mirran Crusader comes in against any BGx deck, such as Abzan or Jund. 4 damage unblockable is a quick clock if it is not removed.