It is called a Jean-a-ma-jig and what a handy little tool.
It only measures 4 mm deep, 5 cm long and 37 mm wide. The cut out portion is 6 mm wide and about 2.5 cm long.
It is promoted as an item to help jump the bumps where seams are bulky, such as a jean hems.
Seriously, this little tool can be used with other fabrics that cause bulk. I'm thinking about that section of topstitching that I was not able to complete on the collar section of the Donna Karan jacket, Vogue 1263.
Unable to jump that hump, I simply lifted up my presser foot, raised my needle and shifted the fabric over the section that was causing the issue. If I tried to tug the fabric from behind the presser foot I would risk breaking my needle or worse damaging my plate cover.
If I had the jean-a-ma-jig I could have successfully topstitched over the bulk.
Here is how it works.
As you approach a section in your sewing that has bulk it has an increased height. The front section of your presser foot begins to raise as it tries to sew through the section.
This is when you want to stop your sewing for a moment and raise your presser foot. Slide the Jean-a-ma-jig from behind over the stitching you have already done to give a lift to the presser foot from the bottom.
Doing sew allows the presser foot to be at a even height with the bulk that you are trying to sew over. It works like a charm. You can now continue sewing.
Some tutorials suggest once the bulky section is sew over that you will want to place the Jean-a-ma-jig in front of the presser boot as the bulk decreases in thickness. I didn't find this necessary and it wasn't taught that way in the sewing class I took.
What was I sewing?
I made a little slip cover for an ice pack. I used a flannel Eddie Bauer fabric and some denim that I had in the scrap pile. The two fabrics were of different thickness and that caused some additional bulk at the side seams. But it was all good with the Jean-a-ma-jig.
I'm adding this little tool to my new top ten list of favourite sewing tools.