When fly casting, a good follow-through is essential for a clean presentation. No matter if you're aiming in close or reaching out to no man's land, a flubbed follow-through can make you miss your mark or even destroy the entire presentation altogether. If your buddies are watching, it can also be pretty dang embarrassing. Who the heck wants that? When shooting line on a cast, this additional step makes everything just a wee bit more complicated—especially for novice casters. Most folks simply let go of the line and let it zing through the guides as the cast is finished. There is absolutely nothing wrong with doing this some of the time, especially on long casts. However, an alternative method of shooting line does have some benefits!
This alternative method I speak of is a pretty simple one, and that is to form a kind of "O" or "tunnel" with your non-casting hand as the cast completes. Rather than letting go of the line completely on the final stroke, allow it to freely flow through this pathway you've made with your hand. You can think of your hand almost like it's an additional and much larger stripping guide for the line to pass through. What are the benefits?
Keeps line in check- If you let go completely when shooting the line, it has more of a tendency to jump around and loop around the rod butt, reel, or reel handle. Forcing the line to first pass through your hand helps alleviate this problem and can also help clear minor kinks and loose debris from the line.
Stop a cast early- As an avid conventional angler, I've been a huge fan of baitcasting reels for many years. These reels offer unparalleled control of the lure for the utmost accuracy. The "O" technique affords the fly angler similarly increased control. Since your hand never loses complete contact with the fly line, the outward flow of line can be instantly stopped or feathered at any moment.
Strip faster- Again, since the line never leaves your hand, it's more of a smooth, seamless transition when you go to strip the fly. It's also slightly quicker since the line is already in your stripping hand and you won't have to reach up for it. As previously mentioned, if you totally let go of the line it can wrap around things when finishing the cast. Having to unwrap the line from your reel or rod butt is not just annoying but it wastes precious seconds and could cost you a fish.
Set the hook- In the off chance that a fish strikes the fly the moment it hits the water, you'll have a better chance of being able to instantly react with a fast and effective hook set.
On shorter and more controlled casts, I'll skip the "O" and just loosen my standard grip a bit to let some line slip through my hand at the end of a cast. It's easier for me to be more precise this way, but I find it much less efficient than the "O" for longer casts.