https://www.pacificbackflow.com/backflow/backflow_operation.php

Might help:

The Wilkins backflow preventer is a type of backflow preventer that uses a double check valve to prevent backflow. The double check valve consists of two check valves that are installed in series. The first check valve prevents backflow from the downstream side to the upstream side, and the second check valve prevents backflow from the upstream side to the downstream side.

The Wilkins backflow preventer is typically installed in a horizontal position. The inlet and outlet connections are located on the top of the device, and the test cocks are located on the side of the device.

The Wilkins backflow preventer is a reliable and effective way to prevent backflow. It is easy to install and maintain, and it is available in a variety of sizes to meet the needs of different applications.