It was bound to happen. Someone placed a tungsten bead in the middle of a soft hackle fly to get it deep in the water column. This is now the most popular of the current soft hackle flies that are available. It may be fished as a nymph or swung as a wet fly, and is especially productive in deeper and faster water in rivers.
It can also be a very productive fly during Callibaetis Mayfly hatches in lakes. Often Callibaetis mayfly nymphs make several attempts to reach the surface before they are actually able to emerge. During these hatches try using a floating line. Let the fly sink, then retrieve it to the surface, and then let it sink again. Be prepared for hard strikes.