A Rouleau Loop is a long, thin length of wire with a loop at one end and a latch hook at the other. It is used for creating a "Rouleau" (a decorative cord of fabric with the seam allowance on the inside).
Fig 1 Fold a strip of bias-cut fabric in half lengthways with right sides together.
Fig 2 Trim the remaining seam allowance to less than the width of the tube section.
Fig 3 Insert the hook end of the Loop turner up the full length of the tube.
With the latch open, hold the fabric at the top then hook it with the latch, making sure that the latch closes.
Gently pull the Loop Turner through the length of the fabric until it appears through the bottom of the tube.
You can now remove the hook and continue pulling gently by hand until the whole length has been turned to the right side.
If you want to fill with piping cord, sew the cord to one end leaving the other end open. Thread the latch down the tube, attach the latch and pull through (this requires a bit of practice).