Wouldn't you have to change the radius to create a "flatter" spot? There are no flat spots on a circle, but this talk of "flatter" spots is more like an ellipse. And the stroke is a circle.
I will defer to the most knowlegeable, but I'm with you for now. If any arc of the foreswing has a flat spot in it, it's a manipulation. And worse, it's a manipulation that must be REPEATABLE. Timed perfectly.
Using my own limited experience, I get the radius of my arc by the Extensor action working on, and extending my left arm. When extended, it holds the constant radius from my left shoulder. Measures the shot, so to speak. If I move or let my left arm bend, impact and low point get shifted.
Maybe if you hold the lag extra long............ Forget it, a flat spot doesn't make sense and I'm going back to the Basic forum where I belong.