Remember that the area outside a trout's cone of vision (we're talking under the water here...) acts like a mirror. This means two things...at some angles, the underside of the water surface will reflect the bottom of the stream (or other things)...and at other angles, will appear to shine as light on a mirror. I think that flash material on the back of nymphs is effective (at least...I catch fish on flashback nymphs)...but I don't really think that it has to do with the reflection on the undersurface of the water. I think that it's more likely that the fish will notice light glancing off the material itself.
Chris
