What this means is that you sew into the seam from the right side of the fabric.  By stitching into the ditch you can hardly see the sewing.  Some of the times you might use this is if you are sewing a band on to a garment.  Instead of sewing on the band, sew into the seam which attaches the band to the body of the garment.

Another time might be if you have a facing and it is loose.  This can be very annoying because the garment will roll on your waist.  You can stitch in the ditch at the sides seams, center back seam and even into the darts front and back if there are any.

If you are doing a Euro hem, you could stitch in the ditch at the side seams to hold the hem allowance up from the inside.

Happy sewing

Judith aka genie