This is a very effective method of presenting a nymph or chironomid using a dry fly as a strike indicator. It is often called a hopper-dropper rig because many people use a big grasshopper pattern as the strike indicator. Google "hopper dropper" and read all about it. This article by Bill is a good sample:
http://social.thefishinhole.com/inde...&articleId=311
The advantages of it are that you are drifting your nymph at a specified depth (the length of tippet between your dry and the nymph is the maximum depth); and your indicator has a hook in it that can also catch a fish.
The disadvantages are that the two flies are prone to tangling up, you can't do it in B.C.
, and you can't really effectively fish the dropper more than 4 or 5 feet deep.