This is a variation on a classic wet fly called the starling and purple (or snipe and purple). In this version, we use holographic tinsel for the body instead of purple thread, and purple dubbing instead of black dubbing.
We also use a slightly different hook shape, for no other reason than we like the way it looks. Try fishing this as a dropper just below a dry fly. It should work just as well in streams as it does when slowly retrieved in still water.