I think the reason the Box Stretch method gives that deviation result that is shown on mzex's screenshot is because the box stretch determines the vertical height changes based on the distance between the two end points of the Beam (modeled by Morph) - check Nicholas' screenshot - and does not follow the curve of the element. This causes the deviation.
So basically the vertical position of each point of the Morph curve is determined by projecting the Morph space curve onto the vertical plane connection the two ends of the Morph.
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