The sling holds the projectile and releases it during the throw. The pouch was made from scrap fabric, cut in a rectangle shape with the two ends folded over and sewn with enough of a gap so the cord could pass through. The cord is tied at each end, bundling the fabric together and making a pouch shape to hold the ball. One end of one cord is attached to a screw eye on the side of the arm. The other is tied to a "quick link" from the hardware store and goes over the pin at the end of the arm.
When loaded, the sling is pulled back tight so there is no slack in the lines. Don't get in front of the trebuchet when it's loaded like this - stay to the side.