This past summer, I was fortunate to be involved with the Youth Fly Fishing World Championships which were held here in PA. This was the first time this event was ever held in the USA. To see this technique in action is actually shocking in it's effectiveness. This technique is better suited to fast moving, riffle water as opposed to slower "frog water", so the possibilities of actually seeing the take are reduced. The broken water surface allows the angler to get closer to the fish without being detected.(They can't see you, but you can't see them either). I find that most of the fish I catch are "felt" strikes, but any unusual movement of the line should be responded to with a strike. Since this is relatively tight line nymphing, not much is needed in the way of a strike. One of the keys to success in this type of fishing is in the use of the proper type of hooks. These chemically sharpened hooks help with hooking the fish, and the shape of the hook helps keep fish on. Two of the best hooks for this type of fishing are Knapek brand hooks, and Partridge Flashpoint hooks. No add on indicators are allowed by competition rules, but the leaders are usually constructed with some sort of highly visible short piece of line that acts as a visual indicator. Here's a link for those folks that would like to learn more about this type of fishing:

http://www.czechnymphs.com/tactics.html