Guild Wars 2

How does this trait work ? http://wiki.guildwars2.com/wiki/Burst_Mastery
What I’m confused about is it’s second part : “Cost Reduction: 33%”.
Does that mean that I will not get damage reduced when using only two bars of adrenaline?
And will the damage be different at three bars of adrenalie with the trait on and off ?

that means if you have full stack of adrenaline and you use your burst skill.

you don’t deplete everything, but you keep one bar and your damage is increased by 7%.

Thanks for the answer!