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.