Salt is not added to the water.
ion-exchange water softners are connected to the incoming water main so that water passes through them, and the water coming out is softened by the removal of Calcium from the water.
The machine does this by having a cylinder containing resin beads which, as the water passes between them, absorb the calcium from the calcium Carbonate dissolved in the water, and replace it with Sodium Bicarbonate (as used in baking powder and indigestion remedies).
When the resin beads are saturated with Calcium, the machine switches over, and passes a strong salt solution through the resin. This forces the calcium out (and is run to the drain) and the resin is recharged with sodium, ready to start the softening cycle again. Any remaining traces of salt are then rinsed away with fresh water, and discharged to the drain. When the machine has finished its regenerating cylcle, it then switches the valves back so that water is taken from the main again, and sent on to the taps.
During the regeneration cycle, the machine sets the valves so that any water used at the taps does not pass through the machine. For this reason a timer is included so that the resultant hard water will only be connected in the early hours of the morning, or at some other time when users will not often be using taps. Some machines have two cylinders, which they regenerate in turn, so that softened water is available from one while the other is in its regeneration cycle.
Depending on the size of your machine, and the amount of softened water used, the machine may have a regeneration cycle at intervals of a week or two. Most machines incorporate a small water meter so that they know when regeneration is due. Some operate on a timeclock which you set according to typical water use.