Observe the dance of the needle as it moves from side to side. This graceful pattern must remain precise and consistent. The needle must move straight down into the material, rise straight up, arc or snap to the other side of the zig zag, and again move straight down. This swing is called "Parabola". If this parabola gets jolted or misaligned, the needle bar will not move as expected. In worst case, the needle may attempt to swing below the needle plate and jam. To correct this, the parabolic adjustment must be made.