This is not a widely known pattern, just one of the best. It was originally designed for a specific application-to imitate the nymphs of blue-wing olives, a widely distributed group of mayfly species. But this fly is a wonderfully consistent producer just about anywhere, anytime. It may be the subtly alluring sparkle of the Krystal Flash body, or it may be that blue-wing olive nymphs are so abundant that they figure into the trout's menu most of the time. Try fishing this deep, as a dropper off a large weighted nymph. It's also an excellent choice to imitate midge pupa.