Ability "can't be resisted" vs Tenacity Up

Debuffs are normally a two roll system. The first roll to see if the debuff is triggered, the second is to see if it's resisted or not. "60% chance to stun" means that there's a 60% chance to even do that second roll. (This is why you don't see RESISTED every time you make an attack and a debuff isn't applied)

What happens with Tenacity Up is it sets the second roll to 100% resist chance. If something can't be resisted, it bypasses that roll and just goes straight to a success.

The same method applies to Foresight and attacks that can't be dodged.