CDC stands for cul-de-canard, the soft feathers from a duck's preen gland near the base of the tail. The barbules are fine and structured, so they trap tiny air bubbles and ride the fly right in the film rather than perched high on the water like an elk-hair caddis. To a trout looking up, it reads as a caddis stuck in the surface, sitting low and helpless, which is what they key on once the fast water settles.
Tie it on in sizes 14-18, tan or olive to match the bugs you're seeing. Tan covers most spotted and tan caddis; olive matches the darker, greener naturals. Fish it dead-drift through clear, low-to-normal currents, where picky trout get a long, clean look at the fly.
This is the fly when an elk-hair caddis draws refusals. The trade-off is upkeep: CDC mats down with fish slime and gel floatant, so don't goop it - dry it with false casts, a dry shake powder, or a patch of amadou and keep the wing fluffed. Best in water temps from the low 50s to mid 60s, when caddis are most active.