Ah Ha. The whole interlocking thread thing is so much clearer to me now.
This is a simplified version of how a sewing machine works. We could split hairs over whether the bobbin case really rotates exactly like this, but I’m not going to. However, let us note that the key difference between my machine and the diagram are the dust balls.
Note to self: Must shake hypnotic effect of the animation and go clean sewing machine.