The fly line is the "line" or "thread" that is used in fly fishing to project a fly into the water. The dynamic is not that of normal fishing techniques where it is a weight on the bait or close to it that allows you to cast, but, thanks to the particular shape of the fly line, you can project the flies of various through the launch called "whipping" dimensions and weight at a greater or lesser distance. The casting dynamic with fly line is similar, as anticipated, to that of a whip, in fact to launch or better whip, by moving the fly rod in a certain way, the fly line is projected upwards behind us, when this relaxes there is the loading of the tip which will collect energy and then transmit it forward when we bring the system in the desired direction.
Warning! This is not a fly casting course but a small explanation of what a fly line is and how it works.