Foot # 97D is a Patchwork Foot, similar to the traditional 1/4" foot, #37. Except for a few important differences. The old 1/4" foot makes it easy to guide your fabric 1/4" (or 1/8" on the inside guide) away from the edge by following the edge of the sole. Both sides of the needle have a sole guide that is 1/4" away from the needle. But you will notice that the foot does not cover the feed dog completely with this skinny seam allowance and may feed a little unevenly. (especially if you have an extra wide 9mm feed dog system.)
So on this new 97D foot, you can see above that the left sole guide is wider, and will perfectly fit over the feeding system for accurate piecing and patchwork. The right guide is still an exact 1/4" on the outside and 1/8" on the inside so you use it just the same way as before. This system takes advantage of the wider feed dog, and now it has pressure on it from above to feed fabrics evenly and straight.
I should also mention that when doing piecing, I like to use the Straight Stitch Plate (See above) with just a single hole for the needle instead of the wider slot. That way the start of your pieces can't get pushed down inside and jam things up.
This foot also is designed to work with BERNINA machines that have the Dual Feed feature. Just engage the dual feed mechanism from the back for even, perfect feeding of your quilt blocks.
The Dual Feed arm moves along with the up and down movement of your feed dogs, grabbing the top fabric in the same way the bottom fabric is grabbed by the feed dogs. Does that make sense?
I love this machine. One of my favorite things about it is that it is so quiet. Even with all these bells and whistles and improvements, it is quiet. And quiet is nice.