What is this weird ugly dark join using mirror modifier, smooth shading, and possibly subsurface modifier? I've made some shape in Blender 2.8 that roughly resembles a handle one might find on a fridge to learn, following along with Gran Abitt's videos.  Where the handle part of the handle meets itself in the mirror, in the "viewport shading" mode, there's an ugly dark band around it.  There is no such join on the backplate.

I think my problem is similar to Mirror Modifier making ugly smoothing on mirrored faces, but the proposed solutions don't work for me - there is no option to "clear custom split normals data", and the problem actually goes away when I tick "Auto Smooth".

The real problem is that I just don't understand what I'm doing.  Why does ticking "Auto Smooth" make the dark band go away?  What is the dark band?  I can't see anything untoward in Cycles whether it's on or off - is it actually a problem?
I should have watched further in the video I was following - there's an errant face in the join left over from extrusions which I needed to remove.



It looks like perhaps "Auto Smooth" selectively applies smooth shading to edges based on the angle between the faces they delineate, and, as the issue wouldn't show up with flat shading, the flat shading applying to the edges at the join was hiding the issue.  (I could be very wrong).