If you think about it, this approach makes perfect sense. The really big trout survive by eating other fish – more so than insects. Of course the size of these big fish will be relative to the average size of fish for every individual stream. So don’t go thinking that you can catch Moby Dick in a pool the size of a bathtub. However by using streamers during a hatch you’ll probably catch the biggest trout in that particular stream.
These bigger fish are predators & so take on all the behaviors of a good hunter. Many times, while a hatch is going on, these big ones will sit on the bottom of the stream watching all the other fish rising to the hatch. They wait & watch patiently, hiding. Then when a rising chubb, etc gets close & is focused solely on the insects, the big fish strikes quickly. This is a lot like a cat hunting a mouse.
The best way to take advantage of this behavior is to use streamers that imitate common forage fish for the stream you’re fishing: Wooly Bugger, Mickey Finn, Sculpin, Muddler, etc. Because different actions will trigger the bigger fish to strike, you’ve got to experiment with different approaches of the streamer you’re using. Try a dead drift approach by letting the fly hang in the drift, giving it a twitch every now & then. Perhaps let the fly bob up & down in the water, just above where you think a bigger fish would hide. Overall you’re trying to make your streamer look like a small fish feeding on the hatch. If none of that works, try putting your streamer right in the big trout’s face. Big fish are territorial & won’t let a smaller fish push it around. By making your streamer act aggressively to a larger fish, you’ll be picking a good fight with the trout.
It’s probably a good idea while using this approach to use a shorter leader of 7 feet in length – or shorter – to help set the hook. When using this technique, strikes will be fast & hard, like a bolt of lightning.
So while it may sound odd to tie on a streamer during a nice hatch (& of course it will take discipline to resist tying on a dry fly) you could very well end up with the biggest fish that the stream can hold.