All one way privacy films work by reflecting light. Essentially whichever side of the film is lighter will be the side you cannot see through, and whichever side is darker will be the side you can see through. Usually this will mean that during the day, when the sun is up the outside of the window will be lighter and therefore the side you cannot see through. Once the sun goes down and lights are turned on inside the building the film will work the opposite way around and you will be able to see into the building but not out from inside.