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. I noticed that my Warcry has "Empowered" in its description. Does it mean that other attacks Empowered by a Warcry bonus receive the Empowered Attack bonus, or just the Warcry gets stronger?
2) Are the said damage bonuses additive or multiplicative?
Meaning, for example:
a) 25% Empowered Skill bonus adds another 10% (from the node) for a total of 35%?
b) 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?
c) Or, alternately, am I taking 10 % of the 25% (2.5%) and then adding the 2.5% to 25% for a total of 27.5%?

Devs, please help. the tooltip here is inadequate and probably should be reworded/clarified.

Thanks.