That's the item, Jorn. Thanks for posting the pic.

For those who have never seen this done, the top of the boom's mast is above the head of the man standing on the load on the truck.

The line being held by the man on the ground is the recovery/guide line attached to the hook. The hook is obviously currently in the bale being hoisted up.

The idea goes like this: The ground man will put the hook into a bale on the ground.  The hook is a nice big shank of steel, perhaps a rod 1/2" in diameter, bent and sharpened into a hook.

The the ground man pulls on the rope attached to a drum bake on one side of the truck axle mounted on the bed of the pickup. The other side of the axle is a spool of line attached to the hook. Brake one side of a conventional diff with less load on the other side, all the power goes to the side with less load -- ie, the bale. The bale whips (and I do mean whips) upward and the guy on top of the stack grabs on, the ground man lets off on the brake line, the bale drops onto the load and the hook is sent back down.

Repeat up to 512 times to load a truck with two trailers with 3-string bales.

The biggest danger I've seen is when the hook pulls out of a bale on the ground.