It's shock cord. The bottom of the caternaries loop around the boom, up the other side and are secured to the black dots, which are probably hooks.

Laszlo

Laszlo,  Got it, so it appears the points where the shock cord is secured are on the starboard side and the little hooks are on port, both of which must be attached to the sail...to add this to my lug sail it appears it would require sewing one or the other on the sail.  Wouldnt work to bring the shock cord down to the boom, would need to wrap around the boom like you described...do you know of any other clever methods easier and quicker than tying reef nettles?

Thanks! Curt

Looks like it is on a sailing outrigger with narrow beam. Likely the Watertribe event. I suspect the boat is single handed.    

   Curt,

My sister uses those looped bungee cords with the black plastic ball on the end for reefing her Main. She keeps them in a mesh bag lashed to the Main mast. When she reefs she just pushes the looped bunge part through  the cringle and wraps it around the boom securing it to the black ball attached on the other end (if I could figure out how to post a picture I would include one so I hope that paints a good enought mental image).

Of course I have to stay "nautical" and lash with a reef knot. . . She is older than me and brags that she is not afraid of the 21st century and I always quip back that when she gets smacked in the nose with one of the bungees she might change her mind.

JP