John,
I just saw your post. I don't know how it got by me. Sorry.
To answer your question, if you just cut a gradual taper the back will be flat in this area. The pushing of the straightedge by 1mm. and then cutting along this line allows the back to be radiused along and across the back. It is basically cutting the shape that a radius dish would put into it if you were sanding the side. Make sure that you mark this side as being "convex" because after you bend it, it is very difficult to see and sometimes you can insert the neck on the sides with the back side on the top. This can ruin your whole day!
I hope this answers your question.
I am glad you are finding the classical building DVD and the techniques I present useful. The method I show IMHO has simplified the process considerably.