This is where 5 minutes with pen and paper would save a thousand words. But here goes. Decrowing is when you shave down the hump of the back of the bow to a flatish surface. In doing so you violate every ring you come across, but when done properly each ring forms a solid line the length of the back of the stave. If you keep those lines parallel and the same width the whole way you will have an unbroken, decrowned piece of wood that should be capable of withstanding stresses as well as a ring chased stave.
Read a good description of what to look for in a board being selected for a board bow and the same principles apply to decrowning. Ideally it would be a nice straightforward process you could do on a bandsaw, but it doesn't work that way. You may encounter hills and valleys, twisting, and sidways zigs and zags just as you would chasing a ring. Follow them, all the while aiming to keep the same line pattern all the way down the stave.
I'm sure others will give you better descriptions, but that's a start.
Ted
