I have yet to fish a flymph, but after reading Wet Flies by Dave Hughes, I was compelled to go for it. It can immitate a number of things, depending on how and where you fish it in the water column. You can fish it in the middle of the column, just under the surface, in the film, or on top as a cripple. Dead drift, or fish on the swing to immitate an emergence! Because of the loose scruffy dubbing, air bubbles will get trapped inside, causing the fly to look like the gas bubbles on a mayfly or caddis pupa as it rises to the surface.
It's a great book for anyone interested, I know a few guys on this site recomended it. I intend on tying a few more color and size variations for myself!