By interlacing the bones with magic, the user is made almost invulnerable to broken bones and blunt trauma. Add the users Magic to his Toughness when resisting damage from blunt attacks.
Because of the skin that gets bruised even as the bones hold, 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 construct or elemental) cannot be fitted with this spellware.