Auto-Knockdown

Fighters can now apply a special On Hit Property called Auto Knockdown using their Crafting Menu for a cost of 2000 gold coins.

The duration is 1 Turn per Fighter Level.

To apply this On Hit Property the Character must meet the prereqs:

Fighter Level 25.
Craft Weapon Base Skill 25.
Melee Weapon Equipped.
Shield Equipped.
Overwhelming Critical in any Medium/Small/Tiny Melee Weapon.
Improved Knockdown Feat.

Once the On Hit Property is applied to a Melee Weapon, the following will happen if the character succesfully lands a hit on someone and all the below criterias have be met.

Criteria:
Check to see if the Fighter has a Shield equipped.
18-20 must be rolled on a 20 dice.
Target must not be Immune to Knockdown-effect.

Saves:
Now the Target makes a Skill Save.
The check is:
19 + BAB + STR Modifier + 1d20 vs Discipline + 1d20.

If the Target fails the save, they are knocked down for 3 seconds (normal Knockdown duration).
If the Target rolls a 20 it's a Auto Resist and the Target will not be knocked down.
If the Target rolls a 1 it's a Auto Failure and the Fighter will get an 'Auto Knockdown: Critical Hit' and knock down the Target.

Since it's an On Hit Property it requires the Fighter to hit her Target for the effect to even being checking, so Concealment and AC will help you avoid being knocked down.