Some Blu-rays also has Picture-in-Picture option, not meny, but they are out there.
In such a case, the first listed that DVDFab automatically chooses for you as being the main feature, is usually just the untouched movie, while the non chosen one further down the list, is the Picture-in-Picture optional choise.
Basicly what happens is, that there is a smaler sized M2TS file, maybe of about 7-800MB in size with a pixel size of maybe 720x480, that is added on top of the 1080p image of the main feature.
In that smaler screen, the director, crew and actors then explain how they did this and that in the scenes you are currently watching. It might also contain storyboards and other stuff that explains how.