Etoile Boost question?

Ok so the wording of the skill makes it VERY confusing. Exactly how does the damage mitigation from this one work? I get it reduces damage taken, but it says that boost decreases overtime, it lists a decrease per second, a max, I'm very confused as to how this works.

As you get hit, the damage reduction goes up, but over time it goes back down. Basically, it is primarily for damage mitigation against immediate consecutive hits.

Ok that makes sense, thanks