Drift, drag, and getting agitated ...
The above is from an interview of Johnny Gomez in Paul Arnold's little gem Wisdom of the Guides.
( Sorry about the rather awful pix. They're the best I could do. )
When I used to fish the South Fork of the Snake down in SE Idaho, and it was my home water at the time, a dark brown rubberlegs stonefly nymph was pretty much a staple for winter fishing. It was quite common for mountain whitefish, and there are a lot of them in that crick, to hit the fly just at the end of the drift when it started to lift, or when it was picked up mid-drift. Lifting the rod tip and tugging the line a bit during the drift became something of a habit when fishing primarily for whitefish.
Trout on the South Fork would occasionally hit the big ole stonefly nymph when it was intentionally moved, but not often enough to think about doing that when strictly fishing to trout.
Moving on to Western Montana and Northern Idaho, especially Northern Idaho, I started noticing more trout hitting the big ole rubberlegs when it was being picked up or intentionally moved - "agitated" as Johnny Gomez called it.
Then I started fishing the extended body JARS and the action on an agitated nymph became more pronounced. To the point that the technique is now incorporated in my approach to fishing the big stonefly nymphs. It is more of a soft water technique than something to do under random circumstances, but it definitely has it's place in fishing freestone rivers in this neck of the woods.
I think extended body nymphs, whether furled or articulated, offer some advantage over regular nymph patterns in that the flexing body movement accentuates the tail movement as well as providing action to the body of the nymph, more suggestive of life than when the fly's entire body is tied on a regular hook, and especially when an extended body nymph is agitated.
P.S. Johnny Gomez was a guide on the San Juan at the time Paul interviewed him. The last comment he made on this subject was that he fished nymphs on the San Juan "About 316 days a year."
The fish are always right.