‘Now in order to flatten the thread you need to remove the twist. You do that by spinning the bobin the opposite way of the spin, which in this case means spinning it counterclockwise.’

 http://globalflyfisher.com/tiebetter/thread_control/

‘After the hook, tying thread is the most important item on a fly tier’s bench.’

http://www.flyfisherman.com/ftb/ssthread/

Twisted & Fat

To be honest, I was never given information re the twisting of thread when I learned to tie years ago. Probably would have been a good thing because I learned with 3/0 thread. I made my way to 6/0 and years later to 8/0, all in an effort to reduce bulk. I don’t recall ever reading about line twist and spinning the bobbin to reduce it. I have, indeed, watched tyers at expos spin the bobbin and understood why, I just never did it. Old dog willing to learn a New Trick. Today, with few exceptions I use 8/0 or 14/0. I am still going to experiment with the affects of twist/no twist.