A seven-foot piece of 1/4 inch shock cord acts as both a daggerboard retainer and a ”JC strap” that holds the sail out in light air downwind. Feed the shock cord through the handle of the daggerboard, around the tack and back to itself, going around the mast and the halyard. In terms of tension, you want the cord tight enough so that the board stays up and the sail out in light air, but not so tight that you have trouble sheeting the sail properly. The seven feet gives you a little extra cord with which to ﬁne-tune the tension.
I forgot about attaching it to the tack of the sail......thanks.....MikeMine is long enough that I pass it around one side of the mast and loop it around the tack of the sail and then back around the other side of the mast back to the daggerboard. The stretch of the bungee then can help keep the sail out when you're on a run in light breeze.
Here's a bit about it from the Sunfish Tuning Guide for Racers (I'm not a racer!):