Requires some technical knowledge and soldering / wiring skills. And Copter 3.4 (or build which can use relay 2), and (2) channels/switches for relay control.
I’ve built these for my brother and myself, and they work flawlessly. I’ve experience ZERO interference issues…however use at your own risk.
I’ve actually been looking at that since I’ve been using one of those on my backup rig. I think it shouldn’t be an issue I just need to look into a bit more.
I’m also looking at a backport interface but it’s a bit more difficult.