How thorns (dual blader) works

Do Arans always get the Thorns effect or is it only if they're below a certain combo? From the looks of it, Thorns only works if your inherit critical success is less than 35%. That's probably why it affects Evans but not NLs/Archers. Maybe for Buccs it applies only if the enemy isn't stunned.

Also, Thorns has suddenly re-sparked my interest in Evans.