Note: "The word "fly" applies to insects that fish feed on. Streamers are not insects, but represents small fishes. They were always categorized as flies in the world of fly fishing."
On some occasions, trout are not looking to the surface for their food supply, they only feed on small minnows.
Streamers are trump cards when nothing else works. You tried dries, wets, nymphs or any other flies that you have and still nothing works. If this happens to you, take out your streamers.
Even if streamers ressembles minnows, trout may take them just by curiosity, just because they are there, and not because they are hungry. If they feed on them because it's what they decided to feed on, hold on to your boots, you are going to have a fine day's fishing. Minnows are active and quite fast, so when trout feed on them they are very agressive and strike like the devil.
When fished properly, up stream and across the river, with a tight line, your streamer will surely be seen by all the sub-aquatic population.
There are many ways to fish streamers and each applies to a certain situation.
In water up to my unmentionables!, I casted a short line, maybe 25-30ft, upstream right in front of me and I let it come down to me, hiting buttom and all the while I was taking the slack in the line. On my second cast I hooked on to a nice 16" brown that was fooled by my streamer. I will never know if the trout took it just because it was there or she was hungry! All I know is that technique has been succesfull!
This technique only works if you fish on a short line, so to have full control over your fly.
Tight lines!