Four Needle Embroidery Machine Janome MB-4 Instructional DVD

This is the DVD that was included with my wife's embroidery machine 12 years ago. This is for instructional purposes, I'm sure there are a lot sold used that this is long gone. This one is headed to the trash.

[@RenBias]

This is the 10th video I've watched and no one has explained anything about the bobbin after it's put inside the door. It just sits there like that? What is it for?