"On demand" when used to describe a water pump, simply means that when there is a demand for water, the pump turns on and provides water. On demand pumps generally activate when a drop in the water pressure is detected. For example, in most instances, the power to the pump is left on, and the water in the pipe between the pump and the point of use (Examples might include: your shower in your home, an outdoor garden spigot, or a dishwasher) is pressurized. When a valve is opened allowing your shower head to come on, the pump detects a drop in the water pressure and begins to pump water. The on demand pump will continue to do so until the shower is turned off allowing the water pressure in the pipes to build. Once the pressure reaches a level recognized by the pressure sensor in the water pump, the pump will stop. The pump's power remains on, but it stops pushing water at this point and waits until another valve is opened before it will start pumping again. See a list of On Demand Water Pumps.