VSL PLUS senses the position of each stabiliser leg, and whether there is a load on the truck.
Variable Stability Limit PLUS (VSL PLUS) is a safety system that automatically regulates crane capacity in relation to the actual stability of the vehicle whilst it is working, by sensing the position of each stabiliser leg, and whether there is a load on the truck. It is designed to protect the crane operator, people nearby, and the crane and vehicle outfit. It gives major benefits in improved safety and stability, by taking the total weight of the truck into account when calculating stability. The system uses the loaded platform of the truck as a counterweight, automatically adjusting the crane's lifting capacity on the basis of optimal stability.
The actual pressure on the stabiliser legs is measured. More load on the truck bed means more pressure on the legs, which means greater stability. So, a load of cargo on the truck will mean that you can lift significantly more before the system stops the crane, allowing the crane to always operate at its optimal allowed capacity. Source: http://www.hiab.com/en/global/about-us/newsroom/releases/releases/?rssid=http://cws.huginonline.com/C/135578/PR/201510/1960319.xml