Fly casting instruction puts a lot of focus on tailing loops, its problems and cure, but almost none on “line dangle” or “dangling end”. I am not sure about the reason for this, as the dangle may be a source of problems on its own (losing heavy nymphs in the grass behind me comes immediately to mind). Maybe it is an issue that hasn’t been addressed specifically because it is considered as a tailing loop?
Anyway, the beauty of looking at a dangle in slo-mo is how it shows the distribution of tension along the fly line: tension is higher in the section of line closer to the rod tip and very low in the line end; so low in fact that it takes some time for that characteristic shape to be pulled out. Does it ring any bell about the tension in the line tip of a D or V loop?