Dynamic spring foot is used for embroidery, and free motion sewing and quilting.  The foot is designed to give a great view so you easily can do stippling or follow a design in the fabric. 
 
The spring action keeps the fabric from lifting with the needle as the stitch is being formed, giving you more control of your sewing.
 
Category J - except Quilt Expression 4.0, Expression 3.5, 3.2, 3.0, 2.0, Expression 150