How does item rarity support gem work?

At the point you hit the monster for damage using a skill supported by this gem, it applies a bonus "drop rarer items" stat to the monster just before damage is dealt, and removes it just afterward, so that the bonus is there when it dies and drops things if the damage from the supported skill kills it.

This can't work like that for DoT, as you can't apply the bonus at the time of dealing damage when damage is dealt constantly. A different method of delivery is planned which may allow us to apply these bonuses to DoT effects in future.