More specifically, the architecture of the multi-link rear suspension provides greater latitude in suspension tuning (coupling between the wheel position and vertical suspension travel) and elasto-kinematics (affecting the wheel position on application of longitudinal or lateral force) than a trailing arm set-up. For example, having flexible bushes on the lower link at the front and stiff bushes on the lower link at the rear induces slight toe-in or toe-out when lateral force is applied, thus greatly improving the stability of the rear suspension under load.
The rear suspension bushes of the C6 also play a role in separating the damping and steering functions.
The radial stiffness of these bushes helps to transmit lateral forces and to maintain the wheel position during cornering. When the suspension moves, the mounts are twisted. The more flexible they are, the less stiffness they create in vertical movement of the wheel. Separating the radial and torsional characteristics of the bushes thus provides improved comfort and roadholding.
The C6 suspension bushes take advantage of the intrinsic characteristics of rubber, a material whose stiffness varies depending on the frequency and severity of the loads to which it is submitted.
The hydro-elastic mounts located under the rear axle cross-member serve to filter and adjust vibration frequencies. 

Intelligent suspension: active suspension with electronically controlled springing and damping
Continuing a long line of executive Citroën models, the C6 features hydraulic suspension with self-levelling, thus allowing good suspension flexibility for optimum comfort.
The suspension relies on cylinders that transmit the suspension load to nitrogen-filled spheres that play the role of air cushions (or springs in a conventional system) to provide support for the vehicle. Dampers limit body movement by restricting the quantity of oil flowing between the cylinders and the spheres.
Vehicle height is managed by wheel movement sensors. Pressure sensors calculate the load on each axle, and the electric pump (integrated with the built-in hydraulic interface) generates the pressure necessary to raise the vehicle to the correct height.
Each cylinder is linked to a wheel sphere as well as to one or two central spheres depending on the axle. These spheres can be isolated by means of a solenoid.
The suspension of the C6 thus features two stiffness settings, as on the C5. The choice will depend on road characteristics and driver input.
This highly flexible and comfortable suspension system changes to a firmer setting to optimize body control during cornering, acceleration or braking. It is a system that combines comfort with exceptional roadholding.
To further increase the efficiency of the suspension system with its new multi-link rear suspension, the C6 gains a second central sphere on the rear axle, thus making sure that passengers at the back enjoy the same high standards of comfort as passengers at the front.
Additionally, the two hydraulic suspension settings are complemented by 16 variable damping settings that are managed independently for the four wheels. Damping control settings are switched at high speed (in 2.5 ms, or up to 400 times per second), thus allowing  the system to control the load on each wheel in real time by altering damping control during vertical movement of the suspension. The system can thus effectively soak up bumps or dips in the road surface.
The suspension control system collects all the information relating to the state of the road and driver input: movement and speed of vertical movement of each wheel, vehicle load, vehicle speed, engine torque, braking pressure, lateral acceleration, steering wheel angle, etc.   Through this information, the system is able to "read" the road, adapting to its characteristics and to driver input in order to improve comfort and body control.
The suspension control unit has two operating modes: “SkyHook” and “RoadHook”.
SkyHook, the main operating mode, places the emphasis on comfort and body control. Using the above data and a model of vehicle suspension, it reconstitutes vehicle movement with reference to a fixed point (the sky), rather than a moving point such as the road.
The system calculates the damping force to be generated to simulate a vehicle whose dampers are linked not to the road but to a point at a constant height that is moving with the car. Using this artificial calculation, the system applies the real-time damping force necessary to minimise body movement in relation to the fixed point and thus in relation to the sky, hence the name of SkyHook.
As a result, passengers have the impression of a "magic carpet" ride.
When the driver adopts a sportier driving style, a second operating mode called RoadHook kicks in. This mode places the emphasis on road holding. The objective here is to apply to each wheel the force necessary to adapt to the characteristics of the road. The switch from SkyHook to RoadHook depends on input from the driver during acceleration or braking. After these transitions, the hydropneumatic suspension will have switched to a stiff setting and damping control is adjusted for each of the four wheels.
The suspension of the C6 features a "sport" position. This allows the driver to choose a more dynamic driving style for sporting sensations.
Some specific situations are processed by special programs in order to make full use of the suspension's potential. 

• An "impact" program detects major obstacles encountered by the front wheels (potholes, speed bumps, etc.) and instantly switches to a higher damping control setting to soak up the vibrations. The system calculates the time elapsing before the rear wheels go over the same obstacle, based on the vehicle's current speed. Damping is thus reduced in order to better soak up the obstacle and limit the effect felt by the passengers. 

• A "wheel bounce" program detects bouncing of the wheels on poor surfaces (speed bumps, …) that adversely effect wheel-road contact and – in consequence – vehicle behaviour, stopping distances and vibration control. This module makes it possible to increase damping only in these cases, without affecting comfort in other situations. This ensures perfect damping whenever necessary. 

This suspension maintains the vehicle at a constant ride height, regardless of the vehicle load and number of passengers and to insure this, body height is monitored  in real time.
In specified conditions, this system can automatically lower the vehicle by around a dozen millimetres from 110 kph, thus optimising high-speed steering as well as fuel consumption.
This suspension requires no maintenance for five years or 200,000 km.