A "brace" of flies is more effective than a single fly and multiples flies tied on droppers are more effective than the same flies tied in-line. A the end of the swing, the rod is raise and the brace of flies on droppers are danced on the surface to induce takes.

The key to me is the term "dead" drift which means the fly is inanimate. Something that does not move can be food or not food.

Movement indicates life and my belief is that the trout interprets the movement as somthing that is living and things that are living indicate things that are food. So the movement attracts the fish's attention. A brace of flies that are moving together, but independently on droppers is more realistic than things that are tied together and move in exactly in line.

The dancing fly(ies) on the surface imitates struggling insects on the water and this indicates something that is vulnerable.