Soft hackle flies have been around from almost the beginning of the sport. They are not a dry fly and not a nymph in most cases. They have a special niche in the sport and many people have neither taken the time to fish them or have forgotten about them totally. They can be a deadly fly in your fly box and there isn’t a wrong way to fish them. Their simplistic patterns make them easy to tie in many different combinations to cover different emergences and patterns.
One of the best times to fish soft hackles are when a hatch is just beginning or coming off strong. They can represent either caddis, sow bugs or mayflies and the soft hackle collar traps air bubbles like the emergers. The most common way to fish soft hackles is to cast them down and across and let them swing in the current. You can also fish them dead drifted under an indicator or dropped below a dry fly. I have even heard of people dropping them off a wooly bugger and slowly stripping back. As you can see there are a numbers of ways to utilize this fly.
To research this fly, a great book by a Bozeman author Sylvester Nemes wrote Soft hackled fly imitations. Also check out this short instructional video by the folks at Rio fly lines on fishing soft hackles. Lots of good stuff here. I always have a selection with me and when the fishing gets tough or the trout refuse my dry fly or cripple patterns I find myself tying on a soft hackle with great success.