Originally by: Yann Xonogoth
Is it applied to the amount of HP added by the hardener, or the totality of the shield HP?
Weeeell… let's get technical here.
Hardeners do not add HP — they reduce the damage applied to the ship by some percentage and makes the HP you have last longer. You've no doubt come across the term "effective HP", which simply means how much raw damage those base HP can withstand after the incoming damage has been reduced by the resists.
EHP = HP / (1 - resist).
The compensation skills increase the resist bonus percentages of the hardeners, and thus further reduce the amount of damage, which in turn means your effective HP goes up.
More specifically, the compensation skills act as a bonus to the bonus — if your skill gives you a 25% bonus, then the hardener becomes 25% more effective. Note, however, that we are talking percent here, not percentage points
. It doesn't add to the hardeners resists value — it is multiplied with it. A 40% hardener with a 25% bonus ends up with a (0.4 × 1.25 =) 50% resist.
Also, note that, due to how resists are applied, an increase from 40% to 50% — a 10pp or 25% increase — in your hardener's bonus does not equate to either a 10% or a 25% increase in HP. Instead, it comes out as a (1/0.5) / (1/0.6) = 20% increase in EHP.
…and this is before we get into the really complicated stuff like multiple levels of resists and stacking penalties to resists bonuses.
So, to answer your question: no, but yes. The compensation skills increase the EHP of your shields against the kind of damage the skill is designed for, assuming you have the right kind of hardeners fitted.