There are many versions of this fly, but the following version & text in italics is taken from Fly Dressing Innovations by Richard Walker (1974):
"You may reasonably ask what is new about this one. The answer is that it is dressed to be suitable for a particular way of fishing it. The head hackles are long and stiff; the wing is of cock hackle that won't soak up water, as does the more usual feather fibre.
The technique is to waterproof the fly with a good silicone dressing and, having cast it out, to draw it fast across the surface, fast enough so that it rises up on its hackle tips, with the wing, body and hook cocked up clear of the water. This will produce a wake very similar to that caused by the natural insects. Sometimes the fly is taken on the move and sometimes while sitting still as the angler lowers his rod and gathers slack line.
The success of this fly and the technique of using it refutes completely the notion that a sunk fly will always catch more trout than a floating dressing fished on lakes and reservoirs.
The dressing is as follows:
Hook: Size 8 or 10 long shank round bend
Body: Three strands of chestnut ostrich herl, twisted and wound over a whipping of silk that has been varnished, while the varnish is still sticky. The rear end has a tip of arc chrome d.f. wool. After winding the ostrich herl, it is clipped as closely as possible to give a velvety appearance to the body.
The wing consists of a bunch of natural red cock-hackle fibres, clipped square level with the hook bend. The head hackle is two long-fibred, first quality, stiff natural red cock hackles.
Exact shades of colour are unimportant. I usually use brown tying silk.
The large chestnut-coloured dressing is generally most useful, but sometimes the same fishing technique applied to a smaller buff version, dressed on a 12 or 14 hook, will kill when the larger fly fails.
It has been found that this pattern also succeeds when fished below the surface, and on rivers trout will take it while it is being pulled upstream."
When the caddis hatch the emergent adult tries to swim as fast as it can to the safety of the bank. In this mad dash for survival it creates a 'V' shaped wake a bit like a small speed boat. The trout are on the look out for this give away sign of fast food on the move and home in on these flies with some spectacular takes. This pattern was designed by Al Troth in Pennsylvania, USA in the 1950's, to imitate the adult caddis in two ways: firstly by copying the shape of the wings and secondly he made sure the blunt stubby wings caused a disturbance on the retrieve, in the water surface that mimics that caused by the scuttering adults. Al Troth used a typical roof wing profile to mimic a whole range of medium to small caddis flies rather than try to imitate a particular species. Al Troth later moved to Montana where he gained fame as a fly tyer and trout guide. His new Elk Hair Caddis Dry flies soon became widely used in the various trout rivers of the Rockie Mountains. It is now used all over the world.
The common or slang term ‘sedge’ originates from the fact that adult Caddis flies can often be found clinging to sedge grass near the waters. Sedge/Caddis flies have four wings. The forward pair are normally a little longer than those at the rear. At rest their wings lie close along the body in an inverted V shape. Caddis flies do not have tails but many have long antenna. The Latin name for this group of flies is ‘Trichoptera’.