Cast When Damage Taken is an inefficient gem to use with Guard skills. Often, small amounts of damage will accumulate into a trigger of the Guard skill, at the least optimal time. However, self-casting the spell is also inefficient, as it's very difficult to predict when you will take heavy damage, combined with the skills' short duration and long cooldown.

This Support Gem intends to fix those issues by functioning as a defensive trigger that makes far more mechanical sense than Cast When Damage Taken: When you reach a certain % of your life/energy shield. It would, theoretically, replace the long-since-useless gem Cast When Stunned Support.

The idea behind the scaling of the gem is simple: level the gem until you are comfortable with how much extra survivability it gives you. Level it too high, and you may die to one-shots at earlier percentages of life/ES. Keep it leveled too low, and it may be stuck in cooldown right when you need it, triggering too often. For Mana Guardians, as well as other builds that have an extremely high pool of life/ES, a high-leveled gem would work best. For builds with very low life/ES, a lower level gem would work best.