In dressmaking the little piece of fabric is used to re-enforce the slit seam. They have a tendency to tear. I used a small piece of iron-on interfacing. Do not use anything stiff as previously described or you won't be able to sew the seam. Your pattern instructions may show a picture of the stay on the wrong side of the pattern piece. I wish I was near so I could show you, not a big deal if you can see how to do it. You can sew it without the "stay" but it may tear at the point of the slit when you wear the garment.