When you encounter another player killing a rare spawn or quest flagged NPC, you can just start to help them kill it. Doing so will increase the health of the NPC by 50% and give you a chance to win loot, completely separate from the other player. If someone else comes along to help out, this will increase the health by another 50%, and give them a chance to win loot as well. You will still get loot with just one hit while the mob is alive, even if combat drops.
This system allows anyone from either faction, in your group, solo, or in another group to take part in killing these NPCs and get a chance at the loot, without affecting your loot chances. For now, it only works for the new rares and big quest mobs introduced in Patch 5.2. Please note that this does not work for normal quest mobs (Kill 20 of X…), only the bigger ones that are usually named and have an icon on their unit frame.
Does Blizzard ever think about the shit they implement?
Because I read “Doing so will increase the health of the NPC by 50%[…]”
and thought “oh hey now you can screw people over by just tapping a mob once and running off.”