Although the shape of the loop is primarily determined by the power snap, the shape of the loop is also influenced by events before and after the power snap.
A world-class power snap has three prerequisites. The first one is the absence of slack. Lefty Kreh expounded the second one: you must get the end of an (aerialized) line moving before you can make a (successful) cast. The third pre-requisite is, if Chris were to describe it, you must get the bend out of the rod.
Casters who lack one or more of these prerequisites can still form a loop. The loops just won’t be world-class.
For example, if a caster fails to get the bend out of the rod before the power snap, during the large acceleration of the power snap, the rod will bend more. The increased bending of the rod makes the rod tip deviate below the intended trajectory, accelerating the line downwards for a few moments. The rod subsequently loads, and continues in an arc… causing the tip to rise… now accelerating the line upwards.
After the loop forms and as the line unrolls, the wave that represents the momentary acceleration down and then up comes forward and the fly leg of the loop crosses the rod leg of the loop to create a tailing loop… or at the very least, a precursor to one.
The loop forms a few milliseconds following the power snap. Once the rod is stopped and the rod straightens, the rod tip begins to decelerate allowing the line to overtake the rod tip. And because the fly line is connected to the rod, the line just outside the tip is stopped, and a loop forms. As the end of fly line continues forward, the rod leg of the loop elongates.
At the end of the power snap, the best casters in the world do three things to keep the loop tight and powerful: they stop the rod abruptly; they dampen the rod extremely well; and, they stop the rod high.
Stopping the rod anchors the loop, allowing the line to turn over with more energy. Dampening the rod prevents slack from entering the system. And, stopping the rod high prevents the loop from being pulled apart.
Movements of the rod tip — after the loop has formed — affect the rod leg of the loop. Perhaps the easiest way to explain this phenomenon is an aerial mend. It allows us to deliver a fly to the target and lay the line down off to the side.
In the same manner as an aerial mend, large counter-flexes… rod tips that fall after making a stop… or rod tips that don’t stop at all… pull the loop apart – even though the head of the loop starts tight.
In addition to preventing the loop from being pulled apart, stopping high serves a second, equally important purpose. Stopping the rod high keeps the rod more vertical, meaning gravity won’t exaggerate the counter-flex as much as a rod that is stopped lower and is more horizontal.
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