Spool Pin Spring -- provides a smoother flow of thread!
Some of the newer thread spools are crosswound which gives them a tendency to wobble on the spool pin while you're stitching.  However, if you put this spool pin spring on your spool pin before your thread spool then your thread will flow more smoothly to the needle.  When winding a bobbin, you still need to go a bit slower with the newer crosswound thread spools, but this little spring will help considerably!