When the violins glissando, they’re the answer to the question posed by his left hand. It’s like he’s squeezing the music out of the air. Then the moment is gone. His left hand is back to supporting the right hand with small, occasional jabs in the air. The violins play thousands of other notes that night. But for those two seconds, because of this little gesture that nobody asked for, the music feels just a little bit like magic.