Thread not only binds materials to the hook it also pushes material around the hook – demo tyers call it ‘thread torque’.
To see what that means, fit a hook in the vice and start thread onto the shank. Cut a few inches of whatever leader material you have (I’ve used matt olive for visibility).
Hold the monofilament in your material hand, so it sits on top of the shank and wind your thread under normal tying tension. I defy you not to simply push that mono over the shank and down the other side. Turn the mono at a 45? angle and you may just catch it – I can’t. If you managed to catch the mono and your thread was a tight as it is when you tie, your thread isn’t tight enough.
Pinch and loop, hold the mono on the hook shank, between thumb and finger of your materials hand, and two or three turns of thread should hold it in position on top of the shank.
Whenever we place a material and wind thread over it, our thread pushes the material in the direction we are winding. The worst way to stop that happening is to wind gently, with slack thread. If your tails are on top when you place them and then mysteriously slide down the far side by the time you complete the fly, chances are you are not using enough thread-tension to hold those tails in place and subsequent tying, dubbing, ribbing or winding the body, twist the dressing and taking the tails with it.
I can almost hear somebody asking if fish care. No, fish don’t care. I care if my flies fall apart, if the dressing twists first cast, if they sit badly on the water, or if they spin rather than swim as I retrieve.