Cast when Stunned is a support gem that causes a linked spell to trigger when the player is stunned or blocks a stunning hit. Cast when Stunned targets the enemy which stunned the player. Spells that don't have targeting, like Discharge or Ice Nova, will be cast on the character as usual. When the character is stunned or blocks a stunning hit, the chance to activate supported spells is rolled individually for each linked spell, then all the spells that succeeded this chance check will activate at once. Multiple copies of the same spell cannot activate at the same time, only one can, however linking multiple copies of the same spell increases the chance that at least one of them activates. After that the global CwS cooldown occurs, which is currently 250ms. Getting stunned during cooldown time will not cause any supported spells to activate, even if you have multiple CwS groups.