I've been with tying and fishing Merkins an bit, and it's occured to me that this fly is really designed to imitate a crab only during the descent to the bottom. If you look at it, it seems those in the know are relly tweaking the arrow body shape and the weighing to get the descent angle and speed just right. Once it gets to the bottom, that's it. It moves a bit in the current, but basically, it's not designed or meant to be stripped.
If the above is correct, then I'm starting to figure out this fly; if not, I still don't get it. Does this make sense to you?