When a mayfly nymph emerges as an adult, it doesn't always escape its old skin cleanly. Many bugs drift downstream for several seconds with the empty nymph case — the "shuck" — still trailing behind. Trout learn this profile (a dun shape with a translucent tail of debris) and key on it during the heaviest emergences.
Patterns that imitate the trailing-shuck silhouette (Sparkle Dun, X-Caddis, RS2) outproduce flush-floating duns when fish are on emergers. The shuck is what makes them look stuck rather than fully hatched.