This bath accessory, that keeps the water level at a safe level, does exactly what it says it will do. There are no electronic sensors or flashing lights, just set the monitor over the drain and fill the tub, when the water reaches the height you set, either 2 or 4 inches, the water begins to spill down the drain. For parents who like to keep a flow of fresh warm water flowing while their babies bathe this is a great device. For safety reasons alone parents felt that this was a good idea to have as a backup to inevitable distractions.