CDC feathers come from the preen gland of ducks; they're naturally water-resistant and trap tiny air bubbles between barbs. A CDC emerger sits in or just under the surface film — the exact "stuck halfway out" silhouette that trout key on during heavy hatches.
The downside: CDC mats with fish slime or water tension. Use desiccant powder after every fish to restore the feather. When tied right, no other emerger pattern catches finicky fish faster.