The context says that this type of axle saves fuel and tire when the trucks have little or no load on them. I think they're referring to air-suspended axles.

Advantages of air suspension systems:
(...)
Higher riding comfort. This gives smoother transport of the goods being transported, no jumping with an empty trailer, and less wear on the road surface.
(...)
Less mechanical wear on components.
Easy to repair and maintain, leading to cost savings.
https://www.haldex.com/en/Europe/trailer-guide/trailer-appli...

Advantages of air suspension
Reduced noise, vibration, and harshness
Improved fuel consumption
https://www.drivingtests.co.nz/resources/advantages-air-susp...

Examples with "air-suspended axle": https://www.google.com/search?q="air-supended axle"