When you cast at any angle other than directly upstream or directly
downstream, you get what's called swing, where the fly swings across
currents instead of in the same direction as the currents.
Quartering downstream is the traditional way to get a fly to swing. As the
fly swings, you want to control its speed by choosing the current you cast
across, by mending the line, or by changing the angle at which you cast in
relation to the current. In trout fishing, you can swing a wet fly, nymph,
or streamer in any kind of water; from fast ripples to slow pools.
Typically, you follow the fly with the rod tip as the fly swings through
the pool. This imitates an emerging insect or perhaps a tiny minnow
swimming across the current. Fish often take just as a fly completes its
swing and begins to straighten below you. At the end of the swing, you can
also strip the fly in a few feet, which may also induce a strike.
In Atlantic salmon and Steelhead fishing, the traditional way to fish for
them is quartering downstream, and strikes almost always come toward the
end of the swing. It's important to vary your swing speed with casting
angles and mends for all species. Too slow and the fish lose their
interest; too fast and they seem reluctant to chase the fly. Catching a
nice trout, Steelhead or salmon on a swing fly is one of the greatest
pleasures in fly fishing.