I saw this on an Achilles 24 today - it looks like a great solution to holding a roller reefing claw in the best place, and also stops it from twisting. The mast end of the timber rod is on its own "gooseneck" fitting
I think the bit of wood stops it twisting as well as keeping it the right distance from the gooseneck, so the axis about which the rollers rotate is kept paralell to the axis about which the boom rotates. A bit of string would just cause it t twist. I think it's quite a good idea. The downside will be that the wear on the sail will always be in exactly the same place.