[rwh]

Why do pros take divots (after impact, of course) during a real shot, but not a practicing swing, yet the amateurs do the opposite (or either no divots or before impact)?

I think the simple answer is because pros aren't practicing or rehearsing taking a divot.

Nor should they. Per 7-6 - BASIC PLANE ANGLES: "So, you never TRY to "take" -- or "not take" -- a Divot. It must be the automatic result of Ball placement AND Plane Angle" [emphasis added].