See the link from littleguest. The important thing there is that it's a rolling window. So your results from exactly 2 seasons ago disappear every week and get replaced by the latest game. If the points that fall off were better than the ones you're getting now then you can drop despite winning. This happens for example if you got far in a cup 2 seasons ago or were beating stronger opponents while now you are just winning games against weaker teams. In national teams it means it's really good to qualify for something since even losses will give better points than points in scrimmages.
Crunchy! If you eat fast enough