Not sure if amplitude is the correct name for the ability. I understand what they are getting at and the metaphor but really it is just the measure of a waves size at any given point in time. If they were to take the derivative of the wave function they would have a spatial probability or a probability function. That would be the function describing the change in amplitude. It is the same logic applied to second derivatives when measuring velocity (acceleration is the change while velocity is the state at any point in time).