I really don't know why it works but it will. I will say that I hook more fish on nymphs than I do dry flies. I fish big weighted flies (#4  hooks) all year unless I'm on a small stream. I fish the same rig in high water or low water. I hook about the same number of fish no matter if the water is high or low. I will say that I hook and land better fish when the water is up. I beleive that big fish feed better when the water is up. I'm not talking about flash flood conditions but when the water is up some with a little bit of color to it. When I fish a small stream I will drop down to a #8 or #10 hook but they will still have some weight on them. I spend 85% of my time on Little River fishing. I fish with a dropper fly, both flies are weighted. I do hook a lot of fish with this rig. As far as fly colors and patterns I stay with Stone Flies, black or brown in color, Hare Ears Nymphs in light brown or a Bitch Creek in black. A Prince Nymph works well. Also use a Tellico Nymph. Hope this info helps for later fishing dates...
May God Bless you... John 8:12