It is a schematic way of identifying a bungee cord. (spring). In reality it is of course just a straight line, but then you won't recognise it easily in the drawings.

The bungee cord actually runs along side the rearbeam from inside one hull to just inside the opposite hull. The Stainless steel ring can slide along and/or extend the bungee cord forward.

I'm using a similar setup to keep my spi sheet tight on my trampoline. That one is just a foot in front of the rearbeam so it doesn't get stuck on the system for the jib sheet. It could however be the other way around.

Wouter