You are correct that the balance characteristics of these vehicles are more complex. Also the "bendi" types are different than swingmasts and turrets. From a training standpoint, I'm not sure that trying to describe it as a trapezoid or moving triangle is going to fly with students that often struggle to grasp the basic stability triangle concept of a standard counterbalanced truck. It does make it just that much more important though that the drivers are absolutely certain that the load is within the capacity of the truck for the height they are lifting it. Beyond that, if the workers understand that when they raise, swing, rotate, shift, or tilt the load, or move or stop the truck, they are impacting the stability of the truck, I think you will have accoumplished your training objectives.