Shield on back in vanilla at the moment adds 20 armor (or 10) to your armor when you get hit there thats it. In RBM we made them block damage fully however we have found out that the hitbox of shield on back is not matching actual hitbox of the shield. Shield held in the hand hitbox is fine though, even if you passively block projectile without holding the block button. It would be also nice if shield could passively block melee attacks, or at least if modders could activate it.