Do you wonder what the black button on this foot is used for?
It locks the foot in a horizontal position when the black button is held in while lowering the presser foot. This provides even feeding at the beginning of seams and helps when sewing over 3 to 12 layers of fabric, such as sewing over welt seams when hemming jeans. During sewing when you reach the point where the layers increase, lower the needle into the down position, raise the presser foot, hold in the black button and lower the foot, it locks in the horizontal position, continue sewing.
Download ZigZag Foot(A) with black button brochure here