The idea that the loop is the element of the cast that moves everything forward is prevalent in the casting world.
Following is a quote from a current conversation on Sexyloops which ellaborates on that concept:
“Because for a loop to unroll there has to be a force pulling the fly leg along, that force come from the momentum change at the loop front. If it stops unrolling the whole thing collapses pretty quickly.”
But, actually, the loop is just the result of the interaction between fly leg and rod leg, and obviously (as Newton’s first law clearly states) the fly leg doesn’t need a force to keep going.
Before loop formation all of the energy and momentum are in the line. The line wants to go on forward due to its own inertia, but after line launch the caster holding the line’s front end forces this front end to turn around: that process is what forms the loop and the rod leg.
The rod leg, by moving slower than the fly leg makes the loop to rotate; the fly leg makes the loop to travel forward in the direction of the target.
Here is an example of the only way in which a loop can pull the fly leg forward:
Actually the loop doesn’t pull forward the line, neither the fly leg nor the rod leg. The key is in the interaction between those both elements of the line: it is that interaction what makes the loop propagate and travel towards the target.
By holding the rod leg we get the necessary difference in speed between rod leg and fly leg for the loop to propagate. The fly leg moves the loop forward:
The rod leg pulling the fly leg forces it to rotate:
The real role of the loop is to keep the fly leg in tension. Without that tension, due to the pulling of the fly leg by the rod leg via the loop, the former can not keep its shape and turns into cooked spaghetti, resulting in another demonstration of the well known flying abilities of pasta:
It is for this reason that we may consider the loop as the steering wheel of the cast: it keeps the line going in the right direction, but the engine of the car is in another place.
Anyway the main use of the loop is never mentioned: to prevent that, after a 20 m cast, the fly fall at the tip of our boots :-)