Period Of Transition
This is a period in which you lose many games and your rating decreases drastically, and it often occurs after a period of steady improvement. The theory is that in order to progress, your brain has to reform itself completely, that is, your whole approach to the game has to change. While that process is going on, your technique can get completely messed up, and you make blunders you haven't made in a long time. Put another way, if your basic understanding of the game is constant, then with experience, you will optimise your technique for that understanding (in fact, this might even hold you back).
There are so many levels of awareness in Go, that this process happens many times, maybe once for each rank. It can be depressing whilst your in it, but once you understand what is going on, it is not as bad as being on a plateau and not improving.
Great article, but uh...shouldn't that be "occurs BEFORE a period of steady improvement"? ~srn347