When proximity sensor control is chosen, the screen of the phone powers down when there is an object close to the proximity sensor. Thus, the screen will automatically power down when the phone is put in the pocket or put screen down on the table. This works regardless whether the user has chosen to keep the screen to be always powered  or whether the screen is normally controlled by the OS. When the phone is away from and object, the screen powers automatically unless the screen has previously been powered down not by the proximity sensor but by other means, such as the OS or a press on the Power Button. 
 
When the screen has been chosen to be always powered, the user can use the phone as a flash light which can easily be turned on by, say, placing the phone close to the chest. This way, the user can shine light for a while to see around, then, quickly stop the light and then, whenever needed, to quickly turn the light on again with just a movement of one hand and not by the power button although the power button will also work as set up. This way, the user would save battery energy as compared to constantly flashing light which may not be needed. The power button still works but the proximity sensor, when used as a switch, is easier to use than the power button. 
 
More information is available in the application.