Just the other day I was contemplating how in the heck does a bobbin work? I've used sewing machines all my life, but never really put too much thought into the mechanics of it all. As long as I could thread the machine and get the stitch width and tension right, I was good to go.
Interestingly enough, I just found a gif of a bobbin in motion that explains it all! Here it is!!
Makes perfect sense now, doesn't it?!