It's called a preventer and it is made of lovely stretchy 3/8" three strand.

So, when the wind gets behind the sail and it wants to jibe, it does. albeit in slow motion as the three strand stretches.

and it cost all of $15 at .50/foot.

I run it from the boom to a snatch block on the rail and back to a winch in the cockpit. Even after snugging it up on the winch, there's still plenty of stretch in it that it will jibe all the way to the other side, but slow enough not to hurt anyone or break anything.