the odds are the the knee bar is for the presser foot, practice with it, it is great when you are chain piecing. You don't have to let go of your work to raise or lower the foot, just leave it down. the only thing with it is that i had to learn to drive with my left foot as I was uncordinated and couldn't do both withthe lame leg, I do have a machine with the knee controler and they are supper easy, you can be way more accurate than using a foot controller
I had brother for years and years and love it, my embroidery machines are still all brother. the shop ticked me off when i went to go get the present machine.