[@Parzivol]

Hmm...I agree with your suggestion on the Atronach sign function.  Perhaps luck may factor in somewhat, as to exactly how much of the spell gets absorbed.  And also things like racial resistances (e. g., Nords resist 50% shock damage).

Hmm...perhaps extremely weak spells could have a chance at being completely absorbed?  Some novice mage that slings a piddly little fireball at our characters, and it's so weak that it just gets totally eaten?  Wouldn't work once you're up against spells that could do some real damage, plus there's always the chance that whatever celestial fates control the absorption will just let it affect you anyway.

I've got a more in-depth theory for how birthsigns and their abilities could work, but that's more for my own personal writing...still, if you'd like to hear it, I'll try to explain it.  It's a work in progress.