Zombie squat or Hands-Free Front Squat can be used as a good exercise to help learn front squat technique or helping fix certain squat issues. It will also work the muscles of the legs, core and upper back. To perform this exercise, go up to the bar on a rack and set in on your shoulders upper/chest with your hands straight out in front of you like a zombie and perform a squat
As you are not using hands to support the bar it forces you to maintain an upright body position, and straight back throughout the lift. If correct position is not maintained the bar will simply roll off.