By interlacing the bones with magic, the user s made almost invulnerable to broken bones and such. Add the users Magic to his Tougness when resisting damage from blunt attacks.
Because of the skin that gets bruised even as the boneshold, this can never do more than reduce the damage to a single wound point for living creatres. Undead have no such limit.
A creature lacking a skeleton (such as a contruct or elemental) cannot be fitted with this spellware.