They prevent static from building up in the bobbin case and that is what causes a lot of the bird nesting from backlash. Changing directions too fast will cause other problems like breaking threads and skipped stitches. I use the Genies in all my machines even the plastic bobbin ones. If I take the paper off the pre wounds I put the thread right on top of the Genie. I have been using them for many years now. They use to be made for industrial machines and just about every floor demo machine had them in the bobbin. I use to have a co worker that owned a sewing factory. He taught me more about sewing machines then I will ever need to know. I would bring old thrift store machines to him and he would go through and explain all about them to me while he fined tuned or fixed them when our work was done. He gave me a bag of the Genies before they were called Genies.