what is experimental film

This is post dedicated to the question "what is experimental film" and I've chosen to take the angle and use my understanding "underground film" here is a quote from a review from and underground film 

"A Rebours is a direct assault to the frontal lobe, an exercise of extreme imagery so intense, it leave the viewer feeling violated, not just via the senses the way a film can affect a person, but physically brutalized. If there were a DVD to transform its audience into a mass of jelly quivering in the corner unable to speak, to feel, to think, to be human, it’s this one. Nothing resembling humanity or “normal” society exists on this DVD. A Rebours creates its own universe and forces the viewer to live in it at gunpoint."

This is a direct quote taken from a review from the page "Underground film journal" it highlights what underground film is to me it is a genre and a class of film making that is shot differently and gives you a different feeling when watching it because to me in watching an underground film you are almost in the film you become the film and the way people respond to certain things they see and hear and to me thats what experimental film is about its about putting the audience in a situation that focus them to react to whats happening in the film and mainstream to me is different its more planned out and has more stages and spaces within the film that gives you time to react underground film is more raw 

Andy Warhol