This is an Ian Colin James pattern. He lists it as a steelhead fly and as one of his favorites in many situations for all manner of trout and bass. His are larger versions. He did have the instructions on his website but I see they are taken down. It took me awhile to figure out (remember) how to tie it! I'm sure those legs move around quite a bit in the water and might trigger a backswimmer response. You can't see it on the photos but I put a small butt tag of pearl mylar.
Since Brian Chan passed around his fly box with flies at least two sizes smaller than mine, I've been on a quest to make my flies smaller. I think that when fish are released, they learn a little about how they were fooled. And so it is important to get a closer representation if you are in that situation. That is not as much a concern around Ontario/Quebec as there are few C&R waters (and consequently a much poorer trout fishery.....so much so I will do more local warm water fishing next year).
The tieing trick are the legs. You can use one pheasant church window or two. I prefer one. Pinch the tip, smooth rest of the fibers back and tie it backwards onto the shank with the stem towards the rear. Dub, then bring the feather forward. Spread the fibers so you can tie down and bring the shell back over. Repeat. You could make a larger thorax at the front or at the middle. I don't think the fish care.
I use yellow or grey rabbit fur at the rear and Black and Red Dazzle Dubbing for the thorax(s). Ian says that all flies should have at least two distinct colours on them. I think he is right.