[Flash Mode] With Leyline of Anticipation, all spells (including creatures) are instants. With this level of surprise, you can spend every turn by untapping your lands, drawing your card, and skipping your turn. Then interfere with your opponent's turn by casting your spells on HIS turn. He can try to attack as if you're defenseless, and then you fire back with a surprise blocker (and potentially kill his attacker) at any point.
[Burn Mode] With Leyline of Lightning, each spell you cast can have 1 point of damage to the player. Multiple ones do more damage (without the need to pay more than the single mana, since it's a triggered ability, not an ability cost). With the amount of cheap spells in the deck, this can be very deadly and kill off your opponent quickly.
NOTE: The odds of getting a Leyline of Anticipation in your opening hand is 36%, and that's not factoring if you're happy with your hand (enough land, etc.). Therefore, more often than not, you will not start out in Flash Mode on Turn 1. There are the other 4 Leylines, so the odds of getting ANY Leyline in your opening hand increases to 61%, which would allow you to sometimes start off in Burn Mode. Even if you don't get one in your opening hand, it only costs 4 mana (though 2 mana are specific), so you could potentially get one (or both) in place on Turn 4.
Most of the creature spells have "enter the battlefield" skills that are very useful for targeting attacking creatures in Flash Mode. Casting creatures in Flash Mode on your opponent's turn also removes most of the impact of Summoning Sickness, as they can block on your opponent's turn, and attack on your turn.
The Renegade Doppelganger works with both your creatures and the "gain control" spells. Splinter Twin works great on all of the ETB abilities. (Note that the Renegade Doppelganger doesn't ETB when copying.)
Dormant Gomazoa is good to cast when a spell is about to target you. It goes on the stack, and is untapped as soon as the targetted spell resolves. Then you have a good 5/5 blocker (or attacker if you really need it). (No, using the LoL to burn yourself doesn't untap it, since it's an ability, not a spell. Sorry.) Skitter of Lizards, while somewhat expensive to multikick, can kill off an attacker of reasonable size as a surprise blocker. Rite of Replication can good the same thing kicked, killing off the attacker and leaving you with four more to use. RoR can also be used to generate tokens for Mass Polymorph.
All of the non-creature spells are sorceries, so that it can take advantage of the better cost/benefit ratios of sorceries and still be instants when in Flash Mode. Act of Treason can turn around attackers to serve as your blockers, or can be used the traditional way your turn. (No, you can't flash/haste attack on your opponent's turn.) Mark of Mutiny should be used on YOUR creatures in Flash Mode as a surprise blocker, since it both untaps for blocking and gives it +1/+1. It can be used traditionally, but only use it when you really need to, or if you're using it as a sacrificial blocker. (You generally don't want to keep a +1/+1 counter on an enemy's creature.)
The opponent's end turn is important while in Flash Mode. If you still have the mana and cards, you can cast the leftovers (if you choose) like you would normally do on your turn. Again, this has the added benefit of getting creatures out without Summoning Sickness, and using Augury Owl's Scry ability right before your draw phase.
The sideboard contains plenty of good counters for various decks. Oxidda Scrapmelter and Manic Vandal adds to the number of artifact destroyers (along with the Demolish already in the deck). The Elementals (Arc Runner and Scoria Elemental) are great against killing larger attackers. Wrap in Flames targets small creature decks, and Fireball is just a good all-around burn card for large/small creatures and players. (Note that the entire sideboard is red mana, so be mindful of your land balance.)