I heard many explanation about how tenacity should work, but everyone had his own version.
1, I think, that according to ingame description it should work following:
Example: your hit does 1000 normal damage and 1000 bonus damage from crit.
Enemy has 40 percent critigation amount, so he critigates 400 of the damage, so you do only 1600 damage instead of 2000.
If you have 50 percent tenacity, 50 percent of the the bonus damage cant be critigated, so the critigation should be only applied on the remaining half. So enemy will mitigate only 40 percent from 500, that means only 200, so the hit will hit for 1800.
2, Some ppl say, that it works different: If you have tenacity 50 percent and enemy has 40 percent critigation amount, he still critigates fully and you need to have critigation over 60 percent to get some effect against such enemy.
Does anyone know, which description is correct? If it was the second explanation, than tenacity is nearly totally useless and works only on really high level vs really high critigation amount.
The second description was correct at khitai release but this was reworked into the first description, after some reports of how useless the stat was considering the actual levels of critigation/tenacity on gear.
So in the end the first description is the correct one.