Toxic growth and "store expected damage"

What does it mean. I shoot pustules with other skills that poison and it somehow stores it? Do I need to max out dps, for example by making em super fast or does it actually need to be real total poison damage.

Expected kinda does no mean anything. Or am i dumdum?

It takes the total damage the poison of the attack would do and stores it.

maybe useful with fast poison attacks that you can't stack enough poisons of but probably intended to turn a high damage single target poison into an AOE.

It also works with gas clouds assuming that they aren't killing all the mobs on there own.