5th line drops the kite instantly without tangles. It's not necessarily safer or less safe than a flag out safety, but it does make it possible to relaunch the kite. I started using 5th lines after I did the swim in heavy surf a few times because I accidentally unhooked and the kite crashed with lines all twisted and I was unable to relaunch.

5th line has the advantage that you can pull in 2 feet of it to fully depower a C kite... on a waroo however we can call it the manual inversion handle.