This is a simple, decorative technique to help you quickly identify center back of your pants waistband.
Make sure waistband piece is cut the size that you need: 2 times the width of elastic plus seam allowances.
With short ends together, sew waistband in a circle creating the CB seam. Fold lengthwise with the seam to the inside and press a lengthwise crease indicating the top edge of the waistband.
Designate one side for the inside of the back of the pants. Stabilize the area to be stitched with tear-away stabilizer or iron on interfacing.
With waistband unfolded, stitch over the seam using a decorative stitch pattern in contrasting color.
Here's a short video talking about this simple knit tip.