We used to call it a dump which reduces the ability of the lift axle to bear the load so enabling more weight on the drive axle. The false economy based on the so called lift axle is not widely understood. The decision to use a lift axle was made in order to save money on tyres at a time when rubber cost more than diesel fuel. I proved back in the 1980s that it costs more to 'carry' the weight of the lift axle in fuel than to drag it, you can prove this yourself by trying to carry your heavy shopping instead of wheeling it. Nowadays it's cheaper to just drag the wheels rather than carry them as it reduces the fuel consumption overall.