


This is currently being included in a new book which a fellow forum member and I are in the process of writing, so I would not want to pre-empt too much.
But broadly speaking, I don't think fish mind a moving fly as long as it just under the surface. An exception to this might be a caddis/sedge.
However, to answer you question.
I deploy a variety of tactics with spiders. Upstream, downstream and anything in between. That is because I work the current with the flies. I don't just let them drift, I tease the fish by subtedly making sure the hackles pulsate. So this is what I do when fishing downstream.
I use a longer leader when fishing down. A 20' leader with three flies is not abnormal for me. I try and keep the line off the water and tease the flies across the currents. I find that the first part of the arc is the most enticing to the fish so I don't often let it swing right around. Let's see if this helps a bit:

Let's assume that we are facing a river with quite a few current in it. I don't try and fish them all, I am selective. I also know that the section C is more productive than sections A & B. So I fish out section C take a step forward and cast again. I continue to work the current until I feel that I have done justice to it and then go back up to the top and step further out into the stream to 2, Then I work current b.
Now if I find that fish are rising, I change to a 12' leader and reduce the flies to 2. I then target specific fish and I place myself upstream from the fish, make a cast that I calculate will allow the TOP fly to drift across and in front of the fish and let the flies swing across. Not too far as will probably pick them up and do it again.
Upstream speaks for itself but I keep the 12' leader and add a third fly.
I could be a lot more specific about each type of current and each type of section of a river but that is for the future.
Hope that helps Otter.