This fly has more action than any other Leech pattern with the possible exception of the Jansen Leech which uses marabou. The added weight of the wire makes the fly bob up and down when retrieved at the right speed. Each time the wire ties the fur down, it creates a bit of a separation. I believe this also aids in the action of the fly.
There is a danger in having too long of a tail with leech patterns as trout tend to sip leeches rather than take them in whole. So you can get some nibbling happening. What you see above is the maximum I recommend.
The colour is a bit off in this photo. The mink I have is actually much darker. The black rabbit version was the best colour but not by much. Browns and olive work well too.
The wiggly look of the backing or hide also gives a 'worm' like appearance. Another aspect of this design is that some leeches have a copper strip down their bellies. The wire hints at this feature.
To tie: After putting on a thread base, start at the back with your thread tieing down the hide, then use the wire. Go four to five turns then repeat ending up at the head. Wetting the fur helps in separating the strands. I use regular floor varathane as my head cement, lightly diluted with water. I also give the underside a coating as it does tend to rotate. The advantage of these water soluble urethanes is that they are translucent upon application and you can see what you have covered. Then it clears.
As far as hook size is concerned, I have been tying my flies smaller and smaller. The above versions are size 10. I tie them as small as size 14.
In cutting your rabbit or mink strips, try for as narrow as you can. The sparser fir strips have more action. My thicker versions have ended up in my bass box (they are less fussy).
Fast and slow retrieves work. I sometimes use two or three different colours in a row to see which one is working better. Always keep multiple flies about 2 ft apart.
Brian Chan says you can use small versions of leeches during chironimid hatches. The trout use them to cork in the tiny buzzers to prevent them from spilling out. They don't have a valve to hold them in the stomach. It is always more fun to fish leeches than chironomids.
I can guarantee you will catch fish with this fly at any time of the year assuming of course that there are trout (or anything else) where you are fishing. Tie them by the dozen. Mine are all beaten and chewed up.