Has it been explained how the kill switch work? I keep watching the scenes with Parrish and I don't see her push any buttons. Was she doing it with her mind or what? This is really bugging me.

It appeared to be done telepathically.

That makes sense. The Marked were nanotechnology, so if they installed some nanos in Isabelle, an electrical field could be generated telepathically to affect Isabelle's nanos. That's just a theory, but I have an impression that since the brain runs on electricity, telepathy must have somethint to do with electricity.

To be able the generate nanos would be a creepy ability if you could send them out and connect to people and read their mind or control them any way you want.