Well “how” a HDR works is it’s a image package containing a series of images captured at X number of exposure settings. The result is an overall lighting solution base on the PBR nature of the material based on the environment in which it was created and not a procedural increase in contrast that is unbalanced and looks more like an increase in brightness.
Awesomeness for film production as the guy responsible for the visual effects can take a series of HDR based images relative to the camera and can easily match up and add their 3d stuff as if done as a practical effect.
How to make use of HDR in UE4 using a post process.
As you can see an HDR will increase or decrease the lighting “balance” but does not effect the lighting energy coming in from the widow.