All DoT damage gives 25% of normal damage threat values.
Shadowknights give 60% of normal damage threat values on dot damage ticks.
The AA adds 10% per rank.
For dot weakening, the enemy will total all the damage it took from DoTs during the last tick and reduce their damage. If you normally do 100 dot damage, the AA just adds a multiplier to that.
Just making up numbers here as an example:
You did 100 damage to the mob, but it counts it as if you did 400 damage.
So if your pulling 700dps in dot damage, your mitigation contribution will be as if you were maybe pulling necro damage values.
The math is weird, and in the end the reduction values on this will look like a good size chunk of DR. I think when I did tests it was 30ish less damage from melee hits from raid bosses and exp mobs alike (the way it scales by HP makes the values similar on small and big alike). It doesn’t seem like a lot, but it was a neat existing mechanic to build upon. Players can probably hit the cap on exp mobs, but you will never hit the cap on raid content.