Now just picture somebody first moving their head up, then down - and compare this with the opposite. I can see different amounts of enthusiasm in these two responses.

I've seen the side-to-side nod. In most of those cases, it was somebody who had decided they would disagree with whatever was said, before it was said, and were just waiting to speak, instead of actually listening.

The first words out of their mouths is usually, "yes, but..." and they proceed to tell you how you're wrong.