Knockout (5e Variant Rule)
From D&D Wiki
The impact from a weapon with the knockout property can cause a target to fall unconscious, without lasting harm. A cosh is an example of this.
If you hit a creature with the weapon and you had advantage on the attack roll, you roll an additional die after the damage is dealt. The value of the die is shown in parenthesis after the knockout property.
If the attack would deal extra damage from a critical hit, or a class feature that deals extra damage as a result of having advantage on the attack (such as a rogue's sneak attack), the extra damage is added to the knockout die (instead of reducing the target's actual hit points).
If the knockout die roll is is equal to or greater than the target's remaining hit points, the target falls unconscious as though it had been reduced to 0 hit points. The targets actual hit points remain as they were.
If the target would make death saving throws, it still makes the throw, but automatically succeeds. The throw is still made in case a 20 is scored.
Constructs and undead are immune to the effect of a knockout weapon.
- Design notes
In the example of the cosh, the weapon deals 1 bludgeoning damage and the knockout die is 1d10. The intent is to allow the classic sap-from-behind against common people with 1 or 2 Hit Dice.
To some extent, the knockout property is a replacement for the 3rd edition "non-lethal damage".