If a hard freeeze is expected we recommend a slow trickle of water about the size of a pencil at several faucets in the house. Cover all exposed pipes. Do not put into bypass. If water pressure is lost leave faucets fully open until pressure returns.
Most water softeners will have a bypass attached to the rear of the valve. It can be a single lever pull or push. It may also be two levers one for the inlet and one for the outlet.