A twin-tube-shock absorber consists of an inner and outer cylinder. The inner cylinder contains oil and the piston rod is connected to the piston valve. Whereas the outer cylinder acts as a reservoir and has a low-pressure gas. One of the advantages of a twin-tube gas-pressurized shock is that less gas pressure can be used to prevent the fluid from foaming.http://www.btshock.com/twin-tube-shock-absorbers/