A large trout comes up and examines your Mr. Rapidan Dry Fly size 12 very carefully but at the last minute he refuses to take it. Maybe it was too large. So you drift a size 14 Mr. Rapidan Parachute Dry Fly over him. This shows him a smaller fly and an entirely different silhouette and light pattern. He comes up and looks at this on two consecutive drifts but he will not take it.
Next you drop down to a size 16 Mr. Rapidan Ant which shows him a smaller fly and a different light pattern. Still–no good.
Finally you drift a Mr. Rapidan Midge over him and he takes it solidly on the first drift. This showed him a smaller fly as well as a different silhouette. I’ll never know why he took it but this tactic usually works for me.
I honestly believe that showing the trout a new fly with an entirely different light pattern accounts for the success of this tactic.