So how does this work? Does it reduce spell effect (ie less dmg) or does it just add magicka? Is it based on percentage or is all magic just reduced 30%?

You know, I'm not sure, but I think that it just let's you absorb the magicka. It seems like ever since I got it I don't take less damage even when I can see the absorption working and my magicka refilling.

Absorb reduces the damage of a spell by 100% if you manage to absorb the spell absorbing the mana used instead.

At least it should work that way as long as the game is not bugged.

You'd think it would reduce dmg because that's how it worked in Oblivion, there seems no way to abuse it in this game (good move Bethesda), and I really would like to not get killed by dragons every time I don't have a ward up :/