The anti-siphon feature is designed to prevent the backwards flow of water from a trough in the event of the supply pressure dropping eg pipe break. Depending on the reticulation of the water on the property this non-potable water (not fit for human consumption) may be drunk by humans. If there are additives in the water this may have serious consequences. There are 2 main methods of preventing this, with the valve mounted above the trough rim or mounted in the trough with an overflow. There are regulations that determine the installation of an “approved” air-gap (distance between the outlet of the valve and maximum water level). Check with your Local Authority. If the valve is being sold as an anti-siphon valve there should be some instructions on how this is achieved.