the red one is normal damper, the lower black line is lower arm, the blue 1 is n2 Gas.
when the wheel move up/ chassic move down, the damper will be compressed normaly, but the N2 gas piston will also compressed, it supply extra pressure to the damper to push it back, when the well move down/ chassic move up, the gas piston will also move down, it decrease the pressure in the gas chamber, it pull the damper piston up & the wheel will move down. that means the distance of the supension moving will be shorter, so the height different between filled up & no fuel of the car will be lesser.