Split thread dubbing is a great way to create brushy natural-looking bodies on flies.
Some threads are better to split than others. Uni thread, because of its construction, can be problematic. UTC and others however are easy to uncord and flatten which makes splitting a breeze. Heavier threads are generally less difficult to split than lighter ones.
It’s a good idea to get your dubbing ready before splitting your tying thread.
Pull down on your bobbin to expose about 3” of thread and give it a counter clockwise spin. This will uncord and flatten the thread so it’s like floss. Run your dubbing needle behind the thread to flatten it even further. Use your needle to split the thread roughly in half and insert the index finger of your left hand between the two strands.
Start placing small slips of dubbing between the strands. Once the split segment is filled, remove your index finger and give your bobbin a healthy clockwise spin to cord up the thread, trap the dubbing and form a spiky rope.
You can then take wraps with the rope to create a fuzzy looking body on the fly.