Pokey, read the first post for explanation. The line is attached to the downrigger by a clip, which pulls loose when a salmon hits.
BA Fisher, I keep the whole shelf in place while driving, with the down riggers parallel to the boat, but with the weights stowed separately. A bungee in each corner, attached to the fiberglass frame is all that keeps it in place. What can I say, it does the job splendidly, and in such a small boat, every part of it can be reached from my seat. If you do something similar, I highly recommend you use swivel mounts for the down riggers.