I'm so glad I stumbled upon this thread. Thanks to the OP for describing it in a way that I understood. I tried this out on a 40 mile ride and it dramatically reduced the pain in the back of my neck. Riding with my back more straight made my reach to the handlebars longer so that my arms were more bent...more so than felt comfortable. So this led me to either get a longer stem or lower the stem (I chose the latter, free option). Now I understand why this posture leads to lowering the stem. Anyway, my neck thanks those who contributed to this thread.
I found this pic on the web that I think illustrates the posture.
