Now that we've cleared up how to use it, a good aside would be about when to use it.
The situation has prolly presented itself quite a few times, but without anything in game throwing up bright flags pointing out the situation, it's easy to miss.
In dungeons, LB is usually the duty of a caster or ranger, because even with only one bar:
AoE LB1 on two targets outdamages melee LB1.
AoE LB1 on four targets outdamages melee LB2.
AoE LB1 on seven targets outdamages melee LB3.
This means it should be used early, on a large trash pull. This will do more damage than using it on a boss, and therefore save the most time.
A melee DD shouldn't waste it on a midboss unless both DDs are melee. Once at the last boss, LB should fall to the melee DD if available.
There are exceptions, of course, but those are the basic rules.
Keep an eye out during the next dungeon after Mettle and you'll find several pulls that are good candidates for a mage to rain down destruction from the sky, and you might even get to LB twice before the last boss.