Melee stats: difference between Damage and Knockdown

Damage

The amount of damage dealt in a single hit. Some enemies are resistant to damage to body. Each enemy also has his own headshot modifier, so headshot damage varies vastly.

Knockdown

This refers specifically to melee weapons and is basically the chance that the weapon has to know the target to the ground. In theory you can have a weak melee weapon like fists and it has a higher chance to knock the enemy to the ground

So in conclusion damage refers to how much damage is caused to a target and knockdown is more to do with the force the bullet/pellet carries

To understand more about what stats mean you can visit the Wiki through this link there is no specific page for melee weapons but a lot of information on all other weapons

If you look on This Page about the fists it explains that they deal less damage than knives but have a greater knockdown effect which is useful for suppresing your target.

Enemies in Payday 2 play an animation when they get hurt depending on how much they got hurt and a bit of random chance. Generally, the more damage you deal in a single hit, the greater the odds that the enemy will experience a heavier, or more disabling, hurt animation.

There are 6 different reactions possible:

• No reaction: The enemy does indeed play no animation at all.
• Light hurt: An animation is shown only if the enemy isn't busy already; he flinches very slightly, virtually unnoticeable.
• Moderate hurt: An enemy staggers for about 0.5-1 seconds but quickly regains composure.
• Heavy hurt: An enemy usually falls to the ground or at least goes on his knees and takes about 1-3 seconds to get up again.
• Explode hurt: The enemy is thrown into the air, falls on the ground and takes 3-4 seconds to start fighting again. Special enemies usually show a weaker animation but are stunned for a few seconds nevertheless.
• Death

When enemies are hurt by bullets, the chance they experience "heavy hurt," or knockdown, depends on the ratio of damage dealt to enemy's current HP. So for example, against regular law enforcers, if you shoot one bullet and deal damage that's less than 30% of his current HP, he'll have a 5% chance to experience heavy hurt. If you shoot one bullet and deal damage that's at least 90% of his current hp, he'll have a 100% chance to experience heavy hurt.

When enemies are damaged by melee attacks, however, the "knockdown" value is used instead of damage dealt to determine the enemy's reaction, and a different table is used to determine the odds of each hurt animation. In the next block, "fraction" is knockdown value divided by enemy's current HP.

Melee vs anyone except Deathwish Murkywaters/GenSec Elites and all specials but including the Taser:

• If fraction < 0.3: 30% no reaction, 70% light hurt
• If 0.3 <= fraction < 0.8: 100% light hurt
• If 0.8 <= fraction < 0.9: 60% light hurt, 20% moderate hurt, 20% heavy hurt
• If fraction >= 0.9: 100% heavy hurt

If a melee weapon has 28 knockdown rating, it means that when you hit an enemy with that weapon, the game compares the value 28 against the enemy's remaining health to determine the odds that he will play each hurt animation. In practical terms, I'm pretty sure 28 doesn't reliably knock down any enemy at full HP.

More information:

Martial Arts basic delivers a 1.5 multiplier to this value, making it easier to reach the stronger animations. In this manner, the fraction may even exceed 1 (technically, the game clamps at it 1 though), yet the enemy survives the attack and usually plays the heavy hurt animation. Pumping Iron aced has no effect on the knockdown value despite the inventory claiming otherwise (Pumping Iron basic is not reflected in the inventory at all). As indicated by the melee table, if the knockdown doesn't reach at least 80% of the remaining hitpoints, the enemy only shows light reactions or none at all. To reliably utilize the knockdown of a weapon, it is mandatory to have a knockdown value of at least 90% of the remaining hitpoints. You can divide your knockdown by 0.9 to obtain the maximum allowed current enemy hitpoints to have a 100% heavy hurt chance. Thus the bat with 240 knockdown can reliably cause heavy hurt when the current enemy hitpoints are less than 267.

Source: Frankelstner's "Long Guide," specifically the "Hurt Animations" section. This information was obtained via his understanding of the game code, read more under "Methods and Terms."