One of the most basic defensive principles is the "force" or "mark". The defender marking the thrower essentially tries to force them to throw in a particular direction (to the "force side" or "open side"), whilst making it difficult for them to throw in the opposite direction (the "break side"). Downfield defenders make it hard for the receiving players to get free on the open/force side, knowing throws to the break side are less likely to be accurate. The space is divided in this way because it is very hard for the player marking the disc to stop every throw, and very hard for the downfield defenders to cover every space.