First, the ball will roll down A, then bounce on the edge of B because of the momentum and bounce perfectly on to the power switch of B turning off the computer because the ball is attached to a elastic string.
The purpose of this device is meant to turn off a computer.
In this video, the ball is being pulled up from a book because of the elastic string it is attached to, gaining potential energy, then falling onto the power switch of the computer with the pull of the gravity and the mechanical energy, turning off the computer.