Each basic attack generates 6 Energize stacks while 1 is generated for every 24 in-game units traveled. This includes but is not limited to simple movement, using dashes or blinks, or being displaced to another location.
The bonus range is applied when declaring the attack, but that attack is not guaranteed to trigger the on-hit effect. For example, if Ezreal'sMystic Shot is fired after the basic attack it is possible for the ability to trigger the on-hit effect before the basic attack hits.
Attacks declared before the charged attack either lands, the target dies, becomes untargetable or if that projectile is destroyed will not generate stacks.
The minimum attack range required for the +150 range cap is 428.57.
Rapid Firecannon's passive stacks with other Energized effects.
The movement speed stat only increases flat movement speed, and will not interact with other percentage increases.
This item allows you to land a basic attack outside of your basic attack range. This is really good for situations where you need to take out a tower but enemies are defending it. You can also use it to poke your enemies from a safe distance, before engaging into a fight, which is especially useful if the enemies are strong enough to engage you if you enter into your basic attack range. Due to those reasons, Rapid Firecannon is one of the most commonly purchased attack speed/crit chance items.
This item is most commonly paired with Stormrazor, which can apply the Stormrazor's slowing passive at a higher range, in addition to both item's energized damage stacking.
It is also commonly purchased by caster ADCs which will purchase Essence Reaver instead of a second attack speed/crit chance item, as they do not really need a lot of attack speed and make good use of the extra range.
Passive - Energize: Moving and using basic attacks Energize you. When fully Energized, gain Firecannon.
Unique passive - Firecannon: Your next basic attack gains 35% increased range, up to a maximum of +150 range and deals 50 − 200 (based on level) bonus on-hit magic damage. Firecannon can be used on structures.