Found this on fishandfishing. Always wanted to try fly fishing.

When fishing is difficult, sometimes only the emerging fly can make fishing possible!

A good emerging fly remains in the water with two thirds of its body submerged in less than a 45 degree angle. It is not unusual to sometimes find that even next to a large insect hathching, especially 24 hour lifespan insects, there is no massive swarming on the water's surface. Trout and graylings seem to ignore all those insects floating on the water, while on the other hand a number of small circles appearing on the surface reveal their presence. Since we are not able to, in those moments, determine what kind of an insect we are talking about, which disables us from choosing the proper fly, we often then conclude that fishing is difficult and that the fish bite poorly!

This is a false assumption, because fish in most cases feed during the entire hatching time, but not on insects already on the water's surface, but those which are undergoing an emerging phase.
This is a transitory phase, between the cocoon phase and the subimago areal phase, when the insect leaves the skin, but still remains in the membrane. Its body is therefore still in the water while its wings are forming and the wing cases barely appear on the surface. Its vulnerability at that moment makes it a truly easy target!

Artificial flies which imitates that phase is called an emerging fly. There are a few very favourable moments for its use: in the middle of the summer, when the beginning of hatching occurs and happens during cloudy and wet weather or, even better, fine rain. Under such conditions, when developed insects appear, in the subimago (dun) and imago (the mature adult) phase, the fish have already overfed on emerging flies.