That is cool that you figured out the mechanics of what was happening. It seems like I read something once that said something similar to this that had something to do with the direction of motion and how the thread was moving either off the bobbin or through the needle. I have a feeling it has something to do with how the thread moved through the slot in the needle- is that the scarf? I don't recall. Anyway, thanks for doing the research, Watson, and the next time I have shredding, I am going to remember this and take careful observations.