How to Empower a Skill?

I found some nodes on the passive tree (in warrior section, specifically) that state: Empowered Attacks deal 10% increased damage.
Questions:
1) What is an Empowered attack? How do I Empower an attack. Could someone provide specific example(s) of such.
2) Are the said damage bonuses additive or multiplicative? Meaning, am I taking some inherent, for example, 25% Empowered Skill bonus and adding another 10% for a total of 35%? Or am I taking the 1.25 (25%) and multiplying by 1.1 for a total of 1.25 * 1.1 = 1.375, or 37.5% damage?

Please help. the tooltip here is beyond meaningless to me and needs to be clarified by the devs.

Thanks.