Since 2011 me and some of my friends have been curating together exhibitions of trash movies, events we have named TrashFests. For each session we establish a criteria and I design a poster based on it.
About what are trash movies, in my humble opinion, it is a cinematographic language which has as characteristics the use of dark humor and being very creative without regards to established standards of quality, what could happen intentionally or not and due to many reasons, including low budget, unreal plots, etc.
And as consequence trash shines more light on the movie making process rather than the product itself, qualifying its flicks as pretty study objects to understand audiovisual elements. And to that I should add that it may be easier to understand the importance of an audiovisual element by feeling its lack rather than by watching its perfection. For example, its easier to understand continuity in a plot that does not have it rather than in one that has, and that is simply because the lack of continuity drives our attention to how important continuity is. And the same logic applies for dialogues, photography, direction, etc.
So, because this language can be easily reduced to its (powerfully creative) essentials, it makes the design process easier and also fun.