It is pretty simple actually. The bottom float switch and top switch are connected in a series circuit. The top swith will always be in the on position so all that is left is for the bottom to fluctuate between on and off with the level of the water in the sump. As the water level lowers the bottom switch turns on and voila the pump hooked to my resevoir send water to the sump until the float switch goes to the off position again when there is enough water. Just in case the bottom switch gets stuck the top switch shuts the system off when the water level actuates that switch. Furthermore you can make sure that you only top off during certain times by adding a timer to the mix. This will ensure that you limit the amount of time the system runs per day and limits the damage any disaster may cause.