Depends on how you read it I suppose.
+ % Mana Skin Gives a % chance that a portion of any damage received may be absorbed into your mana pool, instead of receiving full damage
+ % (chance) ... i.e. 7% chance on your item will take effect of the Modifiers specification:
... any damage received may be absorbed into your MANA pool instead of receiving full damage.
Now, this also indicates that your damage is reduced if the % chance activates.
Now as for the amount or total, that is dependent on the damage taken, and what is returned to your mana pool etc.
Hope this makes sense, I kinda don't get it either, but someone mentioned it previously.