I would summarize as follows, feel free to post your versions:
- from the start position, the rod/line must always be accelerating toward the stop for effective casting
- the first part of this acceleration is gradual to keep tension in the line and keep as much line as possible, optimally all of it moving in a straight vector to the target
- once the loaded rod is approaching the end of it's stroke (the stop point), a burst of instantaneous speed is applied immediately followed by a hard stop, tucking the rod just under the path of the moving line
- this sends the line zipping past the rod tip in a tight loop
The flycast - a thing of beauty indeed.
I agree with the experts who say that extending the stroke (Lefty's rule#4 - giving the cast "help") adds stored potential energy in the lever, increases momentum in the moving line by allowing more acceleration to occur on the way to the final power snap. It sure makes my casts more powerful in wind and distance situations.
Stopping the rod close to the path of the moving line contributes to loop tightness, as does the amount of rod deflection (or lack thereof). A long final burst often causes the loop to open because it causes a large deflection of the rod tip while it's trying to stop. I think this is what Lefty's rule is referring to.
I've also found that stopping high directs any deflection in a more horizontal manner, cancelling out the potential opening effect. Sometime you can see a little zig in the loop from that, but not a zag (vertical expansion, open loop) as does happen when the rod tip deflects downward.
But IMHO the root cause of loop shape is the stop position of the rod relative to accelerated line's path (per Bruce Richards, SA).
Fred/Sean/Jim's descriptions of Lefty's technique reinforces the fact that the final burst of speed is of great influence to the cast, so much so that one can go very casual on the rest of the cast and still get a great cast from a good burst/stop (SUAS).
Thanks to all who have participated, my understanding and ability to describe these characteristics have been enhanced as a result of this exchange.