The Towa "Style L" Bobbin Tension Gauge works by helping you select a bobbin case tension setting for different threads or bobbin styles. This model works with all APQS machines using the Smart Bobbin. Insert a Style L bobbin into its matching bobbin case, and place the case into the tension gauge. Pass the thread over the two pulleys and pull the thread to give you a bobbin tension reading. Test your tension on practice fabric, and adjust the bobbin case tension if necessary. Re-insert the bobbin and bobbin case into the tension gauge and re-check the tension setting. Now with every bobbin change you can fine-tune the bobbin case tension to match the tension gauge setting that gives you the best results on your current project. This part fits all generation models.