Flip Lines of Rope or Steel Wire Core for added safety.
There is plenty of debate over rope only vs. steel core. Yes, you can still cut through a steel core flip line with a chainsaw & even a good handsaw in a couple of seconds if you try. But it's milliseconds without it. And possibly the difference between a nick or completely severed!
On the flip side (yes, lame pun intended!) conductive steel should not be used in close proximity to electrical power lines.
Whatever you use, you will also need a Flipline Adjuster to set the length of it.