The Barbarian no longer loses Fury when out of combat. Instead, he/she will always regenerate 2 Fury per second.
The generation bonus applies all the time, both in and out of the combat.
Using this skill is the most trivial way to get the achievement for fighting with maximum Fury for 5 minutes. However, the credit will only be awarded when players bring the Fury below maximum level after 5 minutes of keeping it maxed out.