I think it’s a theremin (musical instrument). I’ve seen it at school, and you play it with two hands. The instrument is made of a wood block with and two metal poles, one bent, one straight. Don’t quote me on it, but I think the instrument make sound based on how far and how your hands are moving relative to each pole, hence their hands moving and shaking (to make “vibrato”). Search it up. Whether they used it… I’m not sure, but looks like it.
I can't afford to be a patron but I can't get enough of this one. My favorite. Maybe if you do, "I'm Looking Over a Four Leaf Clover" for St. Patricks' Day I might change my favorite.