Headshot from Hunting Rifle, Bow and El Diablo down instantly (without armor).
Headshot from Military Sniper executes instantly (without armor).
A close range blast from Shotgun downs instanly (even with armor, if you hit legs instead of torso).
Close proximity to a Nail Bomb explosion downs instanlty (variable depending on armor and distance from explosion).
Close proximity to a Molotov blast executes instantly (variable depending on armor and distance from blast).
Shiv from behind, or after getting stunned by a Smoke Bomb executes instantly.
Upgraded 2x4 (wooden stick) downs instantly upon melee hit.
If you manage to land a whole burst from the Burst Rifle to opponents head it downs him on the second shot and executes on the third (no armor). Since it also requires one press of R1, I thought it deserves mentioning.
I'm not sure about the Japanese part. lol.
I sure don't do that.
I was instantly executed by bow last night. I wasn't wearing any armor and I think it was a headshot. Some people have the amazing ability!!