I think finishing moves have a chance to happen when no hostile NPCs are targeting the player, or will be after said target's death. Finishing moves happen when the damage from the blow will instantly kill the NPC. My reasons: 1. When sneaking: Slitting throats, I think everybody has experienced this. If the theory is correct, the throat-slitting animation plays because the player has not been discovered, ergo no hostile NPC action. Also, you'll notice that your character plays the standard stabbing animation when the backstab does not instantly kill (which also alerts the target to your presence). 2. Killing the last guy in the group: Last guy in the immedeate area to die (No other enemies will be alerted by this guy's death), therefore no hostile NPC action. 3. Killing civilian NPCs: Again, you haven't been targeted by the city guards or whatever at the moment you click. Killing moves have a chance to happen.