F=MA makes items that apply hit effects, such as Claw of Horus, very strong on the Flash. Basic attacking once and immediately activating F=MA allows you to get all 5 stacks on Claw of Horus in a very short time.
Note that F=MA has a very high Will cost at higher levels.
Mega Rod will fully apply its explosion damage on every basic attack during F=MA.
Hyper Vibration only gives one stack of Speed Force for every champion affected.
Speed Force grants bonus damage not only from his skills, but also items and Stolen Powers. The Flash's Super Speed for example grants him roughly a 10% damage increase.
Speed Storm deals devastating damage over time to all enemies in the area. Since the Flash becomes Invincible during the 2 seconds you can use it when you are under fire, such as to pass turret targeting to a drone or even turret diving enemies.