Oswald writes a nice beginner’s tutorial (intermediate’s reminder) on mending the fly line. The effort is to develop a balance in feeding line up trough the guides and/or adjusting the speed of the fly, via mends, without disturbing the fly and alarming the fish. The correct tension upon the fly line that maintains natural speed (moving at the speed of the current, not speeding up because of drag on the flyline/fly) is a skill that is much more important than how long a cast you can make. It is an important act that is often performed too hard.
“One of the difficulties in mending for most anglers is that their technique eliminates slack in the line and leader, rather than creating slack or moving it to a new position. If the angler starts with no slack, and starts yanking on the line at the rod tip, the fly will move. At worst, the mend will negate casting accuracy by pulling the fly out of the fish’s feeding lane or cause enough drag to spook the target fish. “Mending Your Ways” by Brant Oswald