This is a great invention that has made it into the mainstream. Using your knee, you can lift the
presser foot or drop it at the beginning of a seam. Combined with the needle-stop down, you
can easily pivot when topstitching or appliquéing. This is a real time-saver. Some of these
systems are mechanical, and some have an electronic assist.
One style drops the feed dogs when you activate it. This, combined with the extra lift, makes it easy to slip bulky fabric under the foot. The feed dogs reengage on the next stitch.