The switch to lower it is on the front of the armrest near the floor. The purpose of this is to ease entry and exit from the seat for pax with limited mobility. I'm fairly certain it has to be raised when you're seated as when it's lowered it provides no side protection and someone could fall out of their seat in turbulence. This is likely the same reason why aisle armrests in Y either do not flip up or only flip up with a release button.