Have you ever been stripping your fly only to have a fish follow and turn away at the last second? I've certainly experienced this while both blind casting and sight fishing and it's ALWAYS frustrating! If the fish wasn't spooked by your presence then all is not lost—a quick follow-up presentation before the fish moves too far off may be enough to provoke a strike. The problem here can be with the length of the leader and how far you've stripped-in the fly.
If a fish is giving chase and you strip the leader inside the guides to keep the fly in motion, you run the risk of not being able to quickly make a second follow-up cast. With no fly line outside of the tip-top and leader knots possibly getting hung in the guides themselves, your second-chance cast may not be able to happen in time. Sometimes, you can use the rod tip to pull the fly a few extra feet to either side in hopes of getting a last second grab, but this keeps the fish in close proximity which may spook it. I always try to keep the leader outside of the guides during the presentation which is one big reason why I like to "walk back" my fly at times.
I most often walk back my fly during sight-fishing situations from shore. It's a pretty simple technique that's caught me a lot of fish in the past. Let's say I have a fish following the fly as I'm stripping it in, but my leader is getting close to entering the tip-top guide. At this point, I'll stop stripping and will release the line from my stripping finger, however I keep the rod pointed at the fly and hold the loose line firmly in place in front of me with my other hand. At the same time, I simultaneously walk backwards (often times crouched down for a lower profile) which drags the fly through the water and keeps the presentation going. I may also impart some little twitches with my hand as the fly drags or may even wiggle the rod tip for some added animation. If the fish eats, I can either set with a rod lift or do a kind of strip-set with the left hand. If the fish doesn't eat and turns away, I'm able to quickly re-position and cast on that fish again since the leader and some fly line remains outside of the guides. To make things even easier, if I see a fish following the fly I usually stop stripping and start walking back when I have, say, at least 3 or 4 feet of fly line still out and often more than that. This allows me to make another presentation lightning-fast!
Keep in mind that it may take several tries to get a fish to eat, so the walk back can be performed on multiple presentations to try and coax a bite. Obviously, you can't walk back on just any shoreline and you'll want to watch where you step, but if you have a bit of open space behind you it can be a handy technique. One downside to this technique is that you'll end up several feet further back from where you started and losing sight of a fish is possible, but on the flip side it keeps some distance between you and the fish and away from the water's edge, so it's more stealthy. I've caught a lot of fish over the years using this method and know that it makes me much more efficient in some circumstances!