Trigger gem: Like all trigger gems, Cast when Damage Taken only "casts" spells. Attack gems such as Double Strike are not considered spells. Multiple spells can be linked to a single trigger gem, and will all be triggered if so. Multiple copies of Cast when Damage Taken will also function independently.

Damage counter and cooldown: Each linked spell gains an internal damage counter that tracks the total accumulated damage taken from hits, though not from damage over time or the Blood Magic passive (e.g. you cannot use Righteous Fire to trigger Cast when Damage Taken). All damage counters will be reset to 0 upon entering a new area. When a linked spell is cast, it is put on cooldown for 250 ms and will not track damage during that time. Multiple copies of the same gem share cooldowns, meaning that if two copies of the same spell are triggered within 250 ms of each other, the first will trigger and the second will fail to do so as it is on cooldown. This mostly prevents using multiple copies of the same spell with Cast when Damage Taken, though the use of Cast when Damage Taken gems of different levels can avoid cooldown overlap in some circumstances due to the differing amounts of damage required to trigger the spells. The Cast when Damage Taken gem itself does not have its own counter or cooldown.

Damage taken: The damage taken is applied after mitigations so armour, resistances, and less damage taken of "X" will reduce the number of procs. The damage does not have to apply to life though so damage to ES, mana (through Mind Over Matter), ward, Guard Skills, or other shields like Aegis all count. Shields from other sources such as Frost Shield will not count nor does Immortal Call since it reads as less damage taken.

Maximum level of supported skills: Cast when Damage Taken cannot support skills with a higher player level requirement than that of Cast when Damage Taken itself. Note that this restriction only applies to active skills; any level of other support gems may be used.

Trigger order: Trigger gems and their linked skills are cast in a certain order that depends both on which item it is equipped on and which socket the skills are in.

Damage effectiveness: The damage effectiveness increases with level, but so does the damage taken per cast; overall, the amount of damage that needs to be taken per damage effectiveness goes up at higher gem levels. The optimum is a level 2 gem, which requires 1442 damage taken per 100% damage effectiveness. In comparison, a level 20 gem requires 3057 damage taken per 100% damage effectiveness. This does not, however, take into account the increase in damage from being able to use a higher level gem; for instance, a level 4 Bladefall gem deals an average of 91.5 damage, while a level 20 gem deals an average of 475. On a strict damage-in-per-damage-out basis, Cast when Damage Taken roughly triples in power between level 1 and level 20, depending on the skill used. Any flat added damage will affect this ratio—the more damage added, the less effective it is to level up the Cast when Damage Taken gem.

Targeting: Targeted spells will cast towards the enemy that caused the last damage. Reflected damage is considered to come from the enemy originally damaged. If the damage was self-caused, it will target a random enemy (if there are no enemies, it will be cast at the player, if it is a projectile, it will be fired in a random direction).

Totems: Cast when Damage Taken can't be used with totems.