I'm a little confused by the question. A fly that is drowned, crippled, or spent and is drifting downstream is moving naturally. Generally, a fly is swung downstream in moderately fast, somewhat broken water. A trout, therefore, doesn't have time to be selective, which is why impressionist fly patterns work. Also, even if there is a little under-the-surface drag, there is no wake as there is when there is surface drag. Many angles believe that it's the wake on the surface that turns trout off. Randy