Alan, first point, that is correct. If you move the rod tip well below the straightHobo
I didn't reply earlier because in truth I really didn't understand what you were getting at. I think the penny has now dropped about length of line and casting arc. It's about keeping the rod tip travelling in a straight line rather than an an arc. ? With short line the rod does not bend much so its necessary to have a small arc cast otherwise the rod tip and line will be too low at the ends of the stroke. With longer line length the rod is bent more mid-stroke (I.e. the rod tip is lower) so you can lengthen the arc without the rod tip at the end of the stroke (where the rod is unloaded and hence straight) dropping below where it is mid-stroke. Is my understanding correct ?
Also I think I now understand your pictogram. My initial research around casting seemed to advocate that beginners should try not to use any wrist movement (pivot the rod only at the elbow). You are saying I should be using the wrist also - but only at the end of the stroke. Is that right ?
line path (SLP), you will have too wide an arc and a dome shaped loop. Longer line means more weight outside the rod tip = more bend in the rod. If you stop too soon ie reducing casting arc, the tip will rise above the straight line (SLP) that the rod had travelled to this point and you are likely to have a tailing loop; when there is a bigger bend in the rod, you need greater distance for the rod tip to travel in order to get the rod tip to move through the straight line.
Second point, I use my wrist but not excessively at the ends of the cast but in a controlled manner. I usually use wrist more than other part of the arm on a short cast eg 20ft. For longer casts bigger muscles and more of the arm/body comes into play. You can cast perfectly well with a stiff wrist but it will limit your ability when you start improving. Beginners are told not to use their wrist because they use too much of it and in the wrong points in the cast. If you have too much tendency to use your wrist on the forward cast then consciously try not to use your wrist and squeeze your last 3 digits on to the grip at the stop on the forward cast, I found that advice very useful. It works well when you don't cast with a death grip which we all do in the beginning so much so that I ended up with trigger fingers