While watching the classic concert Paris La Defense, I was reminded of Souviner of China. When Jarre is conducting and sampling the camera shutter click on the mother circular, he presses the white over-sized buttons that appear on the sides of the keyboard. But on the right hand side, there is a black button which has been crudley taped to the side of the keyboard. There is shot where he presses it while pressing the larger white pads to trigger the camera click.
What is it?
Always been curious to know what it is

Jarre concerts attended - Destination Docklands 1988 - Paris La Defense 1990 - Europe In Concert 1993 (Manchester & London) - Oxygene Tour 1997 (Birmingham)
"A group of men and woman entered the laboratory one month ago to live in a smell free world. Body odour, sweaty arms pits and smelly feet are to create the energy to survive. A total mystery surrounds this experiment, and it's project code name: B.O and Smell'i "