Swinging flies for coho IMO is one of the least effective methods for fishing them. I have caught very few the past few years swinging (and I try it nearly every day out). In fact, I think I may be able to count on one hand how many I have caught on the swing.
Then like a fast strip, or if you want to swing, you have to add a twitch in the swing. I achieve this generally by stripping in once every 10 or so seconds and then letting the line slide back out of my fingers the next 10 seconds. I believe this gives the fly a flare in the water, followed my a quick movement (if you're using a swung fly like rabbit or marabou).
Steelhead, as hook has pictures, love to hit swung flies. I find they don't like a whole lot of movement in most cases, they seem to like a slow methodical approach.