Not sure on the light duty wreckers, but on medium duty, and heavy duty units, each sling arm had 4, or 5 holes. While making your hookup, you would pull the pins outward, and turn them to hold out. You would then back up against bumper of the vehicle to be towed, hook up your chains, and secure, then while winch is lifting vehicle up, pins latch themselves into first holes available, as sling arms slide in. hope this makes sense. I could show you better than I could explain it.
