When you are fishing to a trout near the tailout of a pocket or small tool, you've got a presentation problem: the faster current of the tailout will pull on your line, causing the fly to drag. In many situations, that drag is enough to stop a trout from eating your fly.
In this great episode, Dave and Amelia Jensen demonstrate the best ways to avoid dragging flies. The first step involves stealth: you've got to get as close as you can. Then, you use your rod tip to keep as much of the line as possible off the water. The results speak for themselves.