That's the funny-looking silver thing that comes with most new machines, right??? LOL LOL. I was years before I knew what the heck it was for, I just threw it in a drawer and forgot about it.
During a visit, my mother was helping with with a quilt and rummaging thru some drawers she found it and asked if I ever used it. Sheepishly I told her I didn't know what it was. She explained that I used one when I was growing up.
OK, here's the thing. Mom had a very nice sewing machine in a very nice cabinet. When the machine was set up to sew, there was this comfortable, flat lever that you controlled with you knee to move the machine back and forth. I never associated that with this crappy little piece junk lying in my drawer. We both agreed that it was pathetic and wouldn't be used on my machines. It's still in the drawer LOL