What is the purpose of a dishwasher air gap?

Air gaps prevent waste water from flowing back into the dishwasher. An air gap creates an unobstructed vertical space between the water outlet and the flood level of the dishwasher. Unlike check valves and drain hoses, air gaps are able to function in almost all circumstances.

Air gaps are a very effective way of preventing contaminants and waste water from siphoning back into the system. When waste water becomes backed up due to pressure, discharges through the holes of the air gap. Water discharging regularly from air gap points indicates restricted water flow. Most plumbing codes stipulate a number of backflow prevention devices such as air gaps.