Reduced Pressure Backflow Preventer (RPBP) Valve

[Mikealvarado100]

Hi
I know what 'RPBP' valve but do not know how it works? Please explain it's function with picture, if possible.
Does it work like Spring-Loaded Check Valve? it seems it is combined of two Spring-loaded Check Valve, But why two and how are assembled?

 

[Nidum]

http://www.bristolwater.co.uk/wp/wp-content/uploads/2013/03/Diagrams-of-backflow-prevention-devices.pdf