Two weeks ago, in the parc fermé at Bahrain, the Red Bull RB6 was seen to have an extremely high ride height. In fact the car's suspension was holding the car up so high that the rear tyres were in contact with the car's floor, a position that obviously never be useful when running.
After many speculations, it has now been confirmed that Red Bull, and notably also some of the smaller teams' cars have a special ride height control system that allows them to run the cars lower to the ground during qualifying. Since qualifying is now run with the lowest fuel levels possible, and given the fact that a car's suspension has been set up for the race to also withstand high car weights, cars with normal suspension designs are naturally higher above the ground. While this could equal a marginal difference of 1mm, any such difference is vastly important for the efficiency of the car's underbody and diffuser.
Now that everyone has understood the advantages, others teams, including McLaren have confirmed to be anxiously developing similar system which they previously thought were illegal.