Most of us come from a trout angling background and also are made use of to fish behaving in a rather foreseeable way. If there is a caddis hatch going on as well as you can literally see trout proactively feeding for caddis there is a likelihood that if you make use of a pattern copying a caddis you're visiting apprehend a trout. There is a direct and obvious relationship in between exactly what fish are preying on and also what will give you the most effective opportunity for success.
The other popular view on the concern is that they take based on hostility. When they see a little fish in their location they get upset as well as they go after it. With swing flies the fly will resemble its running away the location because of the existence of a bigger killer which's when the steelhead strikes. This is the thought behind Spey fishing.