It's a smack in the back, without assassinating. You both have to be actively jetpacking (So, there's fire comin' out of your pack), and then you hit 'em in the back!
Yeah because of the assassinations in Reach, Bungie renamed the melees.
Regular melee: Pummel
Backsmack: Beat Down
Assassination (animation): Assassination
Paper Beats Rock Defiant: Assassinated a player that just exited Armor-Lock in a matchmade game.
What playlist can I find a Defiant map that has AL?
Premium Battle maybe? I think the objective gametypes in that playlist are based off the BTB ones so they would have armor lock, and obviously the slayer ones would as well.
Not really sure to be honest though.
Also, that achievement is possibly the worst, because you're not supposed to get right up next to a player coming out of armor lock. They can turn instantly as low as they move the camera around in third person making it almost impossible to assassinate them, and you get EMP'd so its a dumb gameplay decision to begin with.
Quick edit, and the go to new post button now working correctly is awesome.