Yesterday I was at God is an Astronaut live performance and was close to the scene.
One of the guitarists (Gazz Carr) performed tremolo picking. But I noticed that on a few songs he used a small machine instead of a pick. It had a blue luminodiode too. So he held this machine over the neck pickups, lighting this luminidiode along the string over fingerboard. It sounded like tremolo picking, but all he was doing was vibratos with his left hand, right hand was immovable.
I want to know how this machine is called (googled a lot — no luck) and, if it not helps to perform tremolos, it's purpose.