Item Rarity + HoI AoE Explosion

Hey, just wanted to ask if the Item Rarity gem applies to the AoE explosion of Herald of Ice. Thanks :)

As far as I know, the skill that actually makes the kill, that is linked to an IIR or IIQ gem will gain the gem's benefit.
Problem is when you use several skill gems and auras - which skill is actually doing the killing?

Wiki suggests adding the cull gem to the linked IIR/IIQ gem and a killing skill so the linked killing skill has a higher probability of getting the kill.

I've given up trying to figure out a way to make this work, and use neither IIR or IIQ gem.