My article two years ago in Fly Fisherman Magazine takes a deeper look at the subtleties of these feeding windows.
I love to swing flies as much as anyone -especially in fall/winter and late spring. But when our Potamodromous "rainbows/steelhead", which "eat, or go through the instinctive motions of such", and are" on the grab", it can be some of the most amazing nymphing you will encounter. But please use caution and don't cast to actively spawning fish, if you take photo opps, keep the female's tail high not to abort eggs, and release fish quickly.
These fish are very A/A..aggressive active, and are fooled easily. Often your quick catch and release will make them passive and more spooked easily, thus saving them from the snagger/igloo cooler brigade that cares less about wild spawning steelhead and their success- sometimes you give them a survival chance by your minimal/educating intrusions into their wild world