M-Class Bobbin Case. Many quilters like to have an extra bobbin case in case one gets dropped and damaged and for use with bobbin work.
If you use the Filtec Magna Glide or Magna Soft bobbins you do not need to remove the backlash spring. If the spring accidentally falls out check to ensure that you have the spring the correct way round and that the slots line up correctly.
Always check your bobbin tension when you change the bobbin by doing the Drop Test. See this video for more information on checking bobbin tension.
You may also want to refer to Jamie Wallen's Tension Video on You Tube.