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I don't know what the exact modifier is, but you get bonus damage and a stun if you are both sneaking and attacking from behind (more to the side and behind, think it's like a 180 angle).
I don't know the exact number, but it's MASSIVE. Considering the difficulty in pulling it off (at least in melee, where it seems two NPCs are always facing each other, so one spots you approaching).... it's well-deserved.