For Iris Caddis, I usually use split-thread to dub the head. Got me wondering about pro & cons for a few other techniques so I tried dubbing loop & touch dub; just rough dubbing the head works pretty well, but for smaller flies (#16 on down), I find mine get a bit too bulky. Since I’m most comfortable with split thread I found that the easiest and fastest to work with (duh!) since the bobbin acts as a self-contained weight and twisting mechanism and no extra tools are needed; only issue is getting the amount of dubbing right - too much and I have to find a way to get that extra dubbing out of the loop to finish the head neatly.
Dubbing loops make estimating loop size unnecessary since I can tie it off with my thread; however, for smaller flies, it adds a bit of bulk and I have to grab a hook of some sort (home-made bodkin) to twist the loop.
Touch dub is pretty easy since no twisting is needed but the dubbing’s not as secure (actually makes the fly buggier if a fish chews on it so this isn’t necessarily a bad thing), and then I have to carry wax in my travel kit (which can melt when I’m out and about on a road trip).
I’d use the same split thread technique. Put the dubbing in, wind the head until it is “full” enough. Then make a loop around my finger with any excess. (Hold your index finger level and below the hook shank wrap the thread with excess dubbing round your finger). Then take a couple of turns of thread around the hook shank. You can now trim the excess dubbing and thread out. The dubbing will come out of the waste loop easily to be re used on the next one.
Another method would be to just act as if you were making a new dubbing loop with the excess dubbing in the new loop. Wrap back and forth over the loop as you would normally in order to close the top. Then cut off the loop.
In effect, this is what Alan is suggesting but it may save a little time.
