What exactly constitutes a rescue kill?

[Papatank]

I've gotten a few of these while questing while NPC's were getting attacked, but never saw one in a dungeon with other players. Does it need their health to be below a certain health, or does it apply while enemies are doing a certain type of attack against someone else?

I'm wondering how easy it is to get a rescue kill for perk planning purposes.

[Karazykid]

I think you have to kill someone who's shooting a shieldless guy. I've gotten 1 and that's what happened for it.

[Valethar]

It almost seems like it's triggered when the person you rescue is close to being incapacitated by an enemy, and you kill the enemy before this happens. I'm not sure I'd waste points on perks that trigger off of a rescue when it seems to be random and very difficult for the player to control.