We load the fly rod and then it (the fly rod) shoots the line to the target and the fly follows, right?
So fly casting is about loading and unloading the rod. That is the "big loading concept", which has been forwarded many times in books, DVDs, articles, lessons and so on.
This video shows that even if I store significant more potential energy in the fly rod by bending it much more than it bends during casting that amount of line (outside the tip) in highest possible line speed, the extra bended (extra loaded) fly rod CAN ONLY shoot the first part of line outside the tip a few meters further than the tip itself. The lines end never passes the tip, nor does it come close to the tips position!
So fly casting is not about loading and unloading the rod in general!
It is about shaping loops, adjusting line speed and trajectory by mainly rotating the fly rod. The fact that the fly rod bends due to rotational acceleration often helps to move the tip almost along a straight path when shaping tight loops. Rod bend simply is the consequence of rotating the rod in order to create line speed.
Only a small part of the energy input by the caster will be stored in the rod when it bends and then only a part of that stored energy will be transformed in line speed when the rod straightens again.
It's fair to say fly casting is done by a controlled rotation of the fly rod in order to produce line speed, shape loops and adjust trajectory. It's not mainly done by loading and unloading the rod. This is only a small piece of the whole process.
Highest percentage of line speed comes by rotating the rod, not by loading and unloading the rod!
A rod does not have to bend at all to create huge line speed with it. At the same time it can bend significant, without rotation nothing I would call to be a cast happens.
Here is an additional video showing me casting a 5wt. fly line with a totally rigid rod (no bend/load at all). You may want to see the very tight and pointy loops as well as a great matching overall line speed. Casts over a hundret feet were possible even though that (rigid) rod had much more weight compared to an average 5wt. rod.
What indeed was difficult/tricky when casting such a rigid rod/broomstick: To achieve an almost straight tip path DURING acceleration when using a wider arc in order to create high line speed for those long casts. Rod hand path has to be adjusted!