The way they described it is if you used synergies to bring down the mob, even with just your own skills, that if your finesse score was high enough from that process the chest appeared. Not really sure if they meant group synergies or even if you were solo this could happen. Either way you get extra loot and thats the key here, no finesse no extra loot.
I understand that part, but why not just add extra loot to the monster/NPC inventory... you loot their body afterwards, not some magical chest that appears out of thin air. As I said, I can understand if that only happens for big boss battles, but not for every rat you kill.
GW2 does something similar, if you use skill to bring down a mob, you get bonus XP, it's very nice. But I hope they keep things nice and subtle... no sense in bragging about an unobtrusive UI, then shattering immersion with dead monsters regurgitating huge treasure chests upon death...