The term medias res comes from Latin and means " in the middle of things". Which paragraphs provide background to the current action of this essay? What is effective an ineffective structuring an essay in this way?

In this question, you have not included information about which essay you are referring to. Therefore, this part of the question cannot be answered. However, we can still look at in medias res in general and at the advantages and disadvantages of structuring an essay in this way.

In medias res refers to a type of structure in which a text begins at a moment of intense action. An advantage of this strategy is that it immediately captures the attention of the reader, as the scene is likely to be an extremely exciting one. Another advantage is that such an opening allows the author to move forward into the future faster than would be the case if he started the story from the beginning.

However, the structure also has potential disadvantages. For example, this structure can be problematic if the conflict is unclear, or if the characters are completely unknown. This can push the reader away as opposed to pulling him in. Another disadvantage is that it forces the author to supply the necessary information about the past through flashbacks and other complex devices. Finally, it can make the timeline of a story difficult to follow.