Cunningham Inhaul
The two photos below show an arrangement that tensions the tack of the sail as you tighten the cunningham. Normally when the cunningham is pulled on the front of the sail goes loose and a large crease appears along the bottom to the clew. This wastes the only bit of the sail that is probably doing any real work and is of benefit in heavy wind conditions. With this arrangement it pulls the tack forwards.
The blue rope has a loop at the end that fits onto the tack tiedown. It then runs through the block tied to the gooseneck and up to the cringle in the sail and backdown to your cunningham cleating point. In this case it runs to the twinning block.
Photos courstesy of Titch
As you can see when the cunningham is tightened the loop on the blue rope pulls the tack of the sail forwards towards the mast and gooseneck.