Experienced fly fishermen do many things in connection with rod handling without giving it a second thought. I been asked asked during clinics why I was throwing the line in a particular way. With experience, you learn to manipulate the fly rod in a number of ways during the cast to meet a variety of fishing demands. You learn to start or stop the casting stroke at different places, use more or less power, bend the whole rod or just the tip, tilt the casting plane forward, backward, or down to a side, or even move the rod off to one side during the cast. Over time, such rod-handling adjustments have been given names such as tuck, serpentine, sidearm, pile, reach, and bounce casts.
Many years ago I learned a valuable lesson from a great fishermen. He told me to make every adjustment you can during the cast rather than trying to mend line on the water. The mends on the water should be minimal adjustments to improve the presentation.
Second he pointed out that keeping your rod pointed towards the fly and parallel to the water is the preferred position. The low tip permits this line to follow the straightest path from the water up through the rod to your hands. This straight path of fly line serves you in several ways. Whenever you retrieve a bit of line , its movement is transmitted to the fly with the least possible delay, which enables you to move it quickly and precisely. This precision is important in setting the hook quickly and efficiently whenever a trout picks up your fly. These advantages of a straight line are lost if you hold the rod tip higher. Line drops vertically from a high rod tip before bending to follow the horizontal layout on the water; this change of direction constitutes slack that can detract from your success. Holding the rod low works toward hooking efficiency in another way. The low tip leaves you the greatest upward distance in which to raise the rod when you are tightening the line to set the hook.
So who was this expert that got me started on the straight and narrow. He was my Grandfather who fished just about every day during the trout season in Michigan.