That "something" is the guard hairs in the fur that's used to tie this fly. They make the fly look 'buggy' & alive. It has a natural movement about it in the water that looks just like an insect scuffling about.
The original pattern was tied by a fellow named E.H. "Polly" Rosborough back in the late '50's or early 1960's. For those of you who haven't heard that name before, he wrote a book called "Tying & Fishing The Fuzzy Nymphs." The title is kind of descriptive & the theory behind the book (if memory serves me correctly) is what I just mentioned: bushier, buggier flies look more alive, & thus are more appealing, to the trout.
There are many variations on this simple pattern & the video below shows one of them. Originally it was to be tied with muskrat fur. The original recipe was something like this:
Hook = #6
Thread = Black
Tail = Clump of muskrat fur w/ guard hairs
Body = Muskrat fur spun onto the thread ("noodle")
Thorax = Muskrat fur in a dubbing loop, or rope with guard hairs everywhere
Head = Ostrich herl, black
Of course, if you don't have muskrat fur, you can substitute other furs, like rabbit. Just be sure that it has a lot of guard hairs in it - the more the better. For a tail you can easily substitute squirrel or groundhog hair - or anything like that.
Over the years this pattern has been given many makeovers & many fly tiers have made it their own by modifying it to suit their needs. You should, too. It can be tied in just about any size you want & since you can use a variety of different animal furs, you can change the color of the fly to match a particular hatch or water condition. *(see below)
Here is a video showing one variation on the original pattern. It does a good job of showing the basic tying techniques & structure of the fly.