Help me understand the principles behind this:

This means that the branch circuits to the range and dryer will carry current in the neutral (they are 3 wire) and then you are guaranteed to have current in the third uninsulated wire from the sub panel to the main box?

If this is the case, don't you have current in this third wire most of the time anyways from the 110 branch circuits out of the sub panel?

This third wire from the sub to the main panel is not insulated like the other two conductors are. It is bare and just enclosed in the woven fiber looking sheath of this "romex" type cable between the other two conductors. How dangerous is this since this wire is normally carrying current?