This is a variation on the cunningham that is pretty typical of some smaller boats. Rather than pull down the cunningham cringle, the entire boom is pulled down to create the necessary luff tension.
There should be a slug or some limit to how much the boom drops when the main comes down, and then when hoisted the main goes right to the top (or the black band if there is one) and then the luff is tensioned by pulling the boom back down.
If the sail is rigged with a cunningham cringle then I'd consider making the boom fixed and using a normal cunningham setup. Just make sure the boom is low enough to allow proper full hoist of the sail prior to using the 'sly pig'