There has been a great deal of speculation on this site as to how exactly flycasting works. For one thing we have smashed the "rod is a spring" theory. Flex an anchored rod by pulling the line against the tip, let go and see just how far the loop travels... not very. Although the effect of the spring is almost negligable try casting a broomstick and you will quickly discover that a flexible rod has a few benefits (if you want to test this out try casting with just the tip section of a rod, then try the same with the butt section).
Here is an animated gif on rod flex...
There are a number of things to be drawn from this video but what I really want to talk about is the flex-to-flex rod that occurs when we make the STOP ("is this the flip-flop?") and the fact that the straight-line path of the rod tip is not the B-all and end-all on the subject as the next image would prove if only you could see the flyline (this is something that I have been hinting at for a little while - ever since I saw a video of me demonstrating at the Chatsworth Angling Fair).
Although one has to take into account the fact that in many styles of fly casting one takes the line around to the side of the rod as opposed to directly over the top (something which becomes particularly important as the loop narrows incidentally) when taking the line over the top of the rod the benefits of trying to perform the flex-to-flex motion (ie the stop) as close to the verticle as possible should become immediately apparent - extended drifting enables this (most of my casts have a Flemish inclination in case you were wondering)
My theory is that the more abrupt we perform the stop the more we force the flex-flex position. How important this is exactly (and this may surprise you) is actually rather difficult to say because no-one really knows precisely how it is that flycasting works...
For one thing I believe that the line passes the rod tip mid-point during the flex-flex position and that this is when the loop is formed (I shall be borrowing a high-speed camera to find out if this is really true) and for another it appears from the forward cast of this GIF that the haul finishes when the rod tip has straightened during the mid-flex position.
We shall have something to say about this :-)
The plan now is to get some clear video footage of loops and completely dissect every component of the stroke. I shall do this in Lapland next week... :-)
A style is born... and we call this style "The Arden Flip-Flop Style"