In the world of wet-fly fishing, you are only as good as your strike indicator. It serves as your eyes, a window to an opaque current through which a submerged hook of fur and feathers entices its target.
Nothing is so essential to success as the tiny piece of yarn or plastic attached to the leader.
If interpreted correctly, it will reveal everything impacting the fly on its underwater travels. Experienced anglers can tell by the subtlest movement if the nymph pattern they have selected has hit a stone, a weed or the maw of a 3-pound brown trout. They know to set the hook only when the latter is at line's end. I, on the other hand, tend to set anytime the indicator changes its course, which isn't bad practice when the distraction is a fish programmed to immediately exhale anything with a hint of artificial flavoring. Most of the time, however, it's a rock or other obstacle, and my flailing only serves to imbed the point deeper.