Sub Skill from Sumonner

Hi All;
I got question regarding sub skill from Summoner.
My main is SpS, I picked Summoner for my sub. My question regards 75 skill - Boost CastSpeed en Power - is it a triger, or counter, when this skill appears:
a) only if samebody hits me, or only when I hit?
b) what is the propability the skill will turn on?

it's a trigger. It's a pretty bad sub skill imo. The increase is minimal

it's a trigger. It's a pretty bad sub skill imo. The increase is minimal

Q: so this skill appears when I hit? or when somebody hits me?

Q: so this skill appears when I hit? or when somebody hits me?

when being hit

it's a trigger. It's a pretty bad sub skill imo. The increase is minimal

the skill is amazing for anyone who takes damage, like a melee or polearm user