It's not for overkilling… I did exactly 20 damage to a Dock Security (who has 20 HP) and got a crushing punch. I got it even though he dodged all damage! Have also gotten it from a single round, so it's not the fact that I used punch twice in a row. Could be something to do with brass knuckles being equipped? I don't recall if I had them on but it's quite possible.
Round 1:
You dive and tumble, your coat billowing.
The guard shoots you for 2 damage, but you catch the worst of the attack in your billowing coat, avoiding all of the damage.You dive for cover.
You punch the guard for 6 damage.
Round 2:
You punch the guard for 6 damage, but he ducks behind cover, which absorbs all of the damage.
The guard dives for cover.You wait until the last moment, then leap into the guard, dealing 14 damage.
The guard shoots you for 1 damage.You've won the battle!
You learned a new Technique: Crushing Punch