randyflycaster,

Yes, upstream mends will add slack to the leader and that is why I stated that on the upstream drift you may only "see" the strike and not feel it. The purpose of the upstream mends is to get your fly down as close to the bottom as you can. You are using floating fly line and if you do not throw a lot of mends into it on the upstream drift, the floating line will be constantly pulling the leader and fly up just like it does when the fly is past you and away. With each mend you can see the leader slowly sinking which will allow the fly to drift down in deeper water closer to the bottom.

I am not familiar with Joe Humphrey's upstream techniques, but, I will read about them because I enjoy this technique very much and always willing to learn more. I use this technique with slow moving as well as fast moving water. Always remember that floating fly line will always hold the leader and fly up and not allow them to sink. Constant mending on the upstream cast allows the leader and fly to sink deeper. With each mend I throw into the drift, I can actually see the leader go down a little deeper. On my drifts, using this technique, the first 3 feet or so of my fly line is always under water and, depending on how fast the water is flowing, there are times I cannot see the end of my fly line. I just constantly watch the line where it enters the water.

I hope this answers your question. If not, let me know or better yet, come fish with me......