Anyone explain the subzero talent

Does frost mage deal 20% more damage to target that is frozen and rooted at the same time, or frost damage increased by 20% if target is frozen or rooted?

so just want to confirm if frost nova is the only way to trigger this effect

It is for rooted and frozen (aka naturally frozen) enemies. So should work with Frost Nova, Ice Nova, Cone of Cold root, Glacial Spike root and supposedly Frostbite root.

Not sure about enemies frozen by Ring of Frost (incapacitated) or Snowdrift (stunned), might test that later with instant Snowdrift.

I was just wondering the same thing, and testing it is kind of scuffed. But im pretty sure it means it cant be a flurry “frozen” it has to be a root “frozen”