To summarize the six most common characteristics of "avant-garde" or "experimental" movies, look into his article Naming, and Defining, Avant-Garde or Experimental Film.
1. Experimental movies are often made with a limited budget and by an individual person, or a small group of investors. Minimal money investments and a limited number of people are key.
2. The small group of filmmakers will often perform a number of different roles within production, which traditionally are assigned to one person at a time. Hence, the writer may also become actor, director, and producer in many occasions.
3. Messages are often more implicit as the storyline is not linear.
4. There is free license as to the use of creative scenes such as slow motion for some, sudden, speedy motion, screen tricks and other special effects that do not occur often, but do add to the uniqueness and experimental nature of the film.
5. Its style aims to go completely against the style and norms of traditional, commercial productions. Hence, we can conclude that experimental filmmakers pay close attention to patterns and trends in commercial movies to go completely against their style.
6. Creates situations where a myriad of conclusions can be reached and there may even be moral ambiguity.
These six traits are repeated from the book into the article for Frameworks. These are the traits paraphrased and explained a bit further.