Winter steelhead often hold in slower currents than their summer cousins. Many flies appear stiff and dead as they swing through water like this, limiting their ability to tempt a strike. A generation back, winter steelheaders began using marabou or rabbit strips to help give flies slow-water action, and these flies were definitely a step in the right direction. But marabou and rabbit have the opposite problem from the stiff flies: they're so supple that they will fold back in a run with even a moderate current--giving the impression of stiff, lifelessness.
These days, you'll find winter steelhead guides tying flies with several different materials, i.e. marabou, ostrich, peacock, Rhea, deer hair, saddle feathers, etc. Sure these diverse materials create beautiful flies, but beauty isn't the reason for the diversity. Each of these materials flexes at a different current speed, thus lending the fly undulating action in slow, moderate, and fast currents.
Personally, I use stiff materials under softer materials to help bulk up the body of the fly. Also, I use stiffer materials at the head of the fly to create that fishy front-wide tail-narrow appearance.
The trick, I think, is to create a thick fly that will undulate in a range of currents, and yet remain easy to cast.