Backflow (cross-connection) is the flow of water and other substances into the potable supply from any source other than its intended source. In other words, it's when potentially hazardous substances get forced or sucked into the drinking water system. Backflow is caused by backpressure (pressure on the house side of the meter is greater than the supply side) or by backsiphonage (negative pressure causing substances to be sucked into the supply lines). Backsiphonage (the effect is like a soda straw) can occur when a leak, line break or heavy usage causes the pressure in water lines to drop below the pressure in other areas of a home. Both cause a reversal of normal water flow, possibly drawing contaminated water into the public water supply system.