In regard of loop formation and loop shapes and sizes I several times met fly casters who indeed tried to convince me that they "can cast a much smaller loop when side casting, because the upper loop leg does not fall that easy on the lower one". Well, they obviously had no clue what loop size actually is about. The loop size is defined by the distance of the upper to the lower leg in a vertical position. If you cast sideways it is more or less the distance from the rod tip to the upper leg measured in a 90° angle.
I call this false attempt of casting a smaller loop nothing else but casting a cheated loop.
And YouTube is full of them! Such a loop might look quite small if you look at it from some distance from a 90° angle to the casting direction but if you place yourself in some distance behind the caster and look at his loops from behind in a line with the casting direction you will see their real size. Such pretty-looking loops which the inexperienced caster would eventually call sexy can turn out to be easily up to 2 m in size if you look at their real size from behind.
When teaching well advanced fly casters I always force them to shape down the loop to the minimum by casting vertically. By doing this the loops turn into real loops (in regard of their size) so you can easily see your progress.
If one wants to move on to get closer to master like fly casting one has to reduce the input by still getting the same result. This means a perfect presentation to a target with the least possible input that would finally result in significantly fewer spooked fish. To reach this goal the casting speed has to be reduced and the loop has to be tightened. If one performs that exercise correctly, there will eventually pop up a question which I tried not only to explain but also proof with an experiment that we made in a laboratory specialized in aerodynamics in 1999.
The experimental arrangement should reproduce the wanted effect continuously. Therefor two pieces of knitting wool in well visible colours were put through an iron ring and knotted end to end. That endless loop was blown at by a very pointed air beam positioned behind the centre of the iron ring so the loop was speeding up. The higher the speed the smaller the loop got. Suddenly the wanted little waves showed up (pictures). The shape of the loop could be changed by altering the pressure of the air beam or creating a wall from the front side with your hand.
What is the reason that causes theses waves?
The waves are created when the rod reachest the furthermost position and flips back, so the upper leg overtakes the lower one. So what we call a loop is nothing but a wave that runs from the rod tip to the end of the line. The upper leg of the loop of an enrolling fly line always needs to move faster than the lower one in order to keep the line enrolling. This is in my opinion one of the most important fly casting principles. If you now go down with the casting speed and keep the loop as tight as possible it gets more and more difficult for the parts of the fly line to move around "the invisible wheel" that keeps the line unrolling. The tractive forces of the fast mooving last part of the fly line start fighting with the stiffness of the material (coating). The speeds of the two legs finally reach a critical point and the upper line starts pushing against the parts in front of it. The last meter of the fly line starts to crimp. As the speed of the upper fly line is much faster now the slow wave gets splitted into smaller ones. I call this the accordion effect.
To get rid of these little waves one just needs to widen the loop a bit (slow down the speed of the upper line) or to speed up the whole system a little (power input). Both helps. Especially triangulated fly line tapers and such with very thin ends flatter more than others if you perform this exercise.
You might ask now for the reason to cast such extremely small loops. Well it is a good training exercise on one hand which helps you to cut a front wind better if necessary. On the other hand it is a perfect one to get better line control, too. You will also find out about the minimum loop size for your fly line or if the front taper of the fly line was not cut correctly and is a bit too long.
There is only one negative point: You will work down your fly line much faster as it gets more and more micro cracks. Smallest loops are line killers, but they also help you to make your line lose memory. It gets dead -as we used to say- much faster. So the exercise has its good but also has its bad to say it in Bono Vox’s words but in any way the little waves point to the fact that you are on a good track to become a better fly caster.