That would be the autopilot disconnect button or, in this case, the trim switch, the pressing of which is the correct immediate response to this occurrence, followed by the disabling of the system using switches on the center pedestal. While it's certainly hazardous for Boeing to knowingly expose pilots to this occurrence, I'm also conflicted by the fact that they failed to respond properly. The procedure is similar on Boeing, Airbus, Embraer and Bombardier aircraft. Standardized, in a way.