Join Date: Jun 2008
Location: Oregon Steelhead Water
I understand your confusion and this raises an important point. In reality wind does not always blow straight upstream or straight downstream - it often blows cross stream, diagonal up, diagonal down, straight toward you, etc, then changes direction! This is why learning as many different casts as you can over time is valuable.
As a beginner, be patient with yourself, get an instructor if you can. Since I live in Hawaii, I am totally self taught with the huge help of instructional dvds like the Art of Spey Casting and others. I have probably developed some bad habits, but at least I get my fly to the fish and just enjoy spey casting regardless, so I am happy!
Back to your question, a down stream wind is one that is blowing in the same general direction that the river is flowing. In general, you want to use a cast that places the d-loop on the downwind side of your body so the wind is not blowing your d-loop towards the body, but away from the body. Over time and with practice and experience, you will learn how to instictively choose the right cast for the wind direction and other conditions as well as being able to tweak your technique for those sometimes minute breezes that start blowing your casts.
As you become more profiecient with your casting, you will find situations where you start "bending the rules" and still getting the job done. A good example is what Derek Brown calls the square cut (also demonstrated on the Art of Spey Casting by one of the guys from Japan). This is essentially using a single spey in a downstream wind. You get away with this cast by placing the d-loop off to the side and away from the body. Another thing, I tend to tilt the rod off to the the side of my body most of the time during my forward casts - this helps ensure that those sharp hooks stay in the air/water and not my flesh.
Sorry to overload you!