When making the locking stitches at the beginning and end of my stitch line, I get the bobbin thread doubled up and wound around the bobbin (I think). When I go to pull the finished sewn piece out, I'm pulling 4 threads instead of 2. Once I cut the ends, I can pull the looped thread out. When stitching fwd then rev, then back fwd again when is there a correct time engage this. Like needle up or needle fully down? How many stitches do you use to do this?