The Pro-Troll Black Box regulates the electric charge on your downrigger cable.  Whenever you put a boat in the water a natural electric charge is generated around the boat.  The different metals on the boat react in the water and a weak battery is created.  Your downrigger cable picks up this charge.  If it is too high or two low, fish will be repelled away from the boat.  With a Black Box you can dial in the correct charge for each species.  With the correct charge, fish will swim right to your cable and you will have a much better chance of catching them.  The Pro-Troll Black Box hooks into your boat power and a contact sleeve is run to each downrigger cable.  If you are not using downriggers you can install the Pro-Troll transom mount and accomplish the same thing.