Product details:

The BrassCraft 1/2 in. x 3/4 in. Anti-Siphon Hose Bib Vacuum Breaker prevents accidental siphoning of hazardous material into the plumbing system when there is a loss of pressure due to a power outage. The vacuum breaker has a small valve that opens when the pressure drops, allowing air to bleed into the system. Easy to install with minimal fuss.