If its the button behind the gear change lever then it tightens up the electronic sway bars automatically so you have less body roll, the x5 stay flat going round & thru corners. The harder you drive around corners the more tension is applied to the X5 level.

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X5 M50d White | 20" Rims | Adaptive Drive, Self-Lvl Susp | Act/Steer | HUD | Xenon, Adapt Lights | H-Beam Ast | Rear/Side Camera | Panorama S/roof | Auto Tailgate | Comfort, Active, Ventilated, Heating, Lumbar, Navada Perforated Leather Seats | Comfort Access | M-Steer Wheel w Pdle Shift |Sports 8-Speed | Active Crus Stop/Go | Proff Nav + TV, DAB | 16 Spk Ind Audio | USB |Internet-eMail-SMS-Apps | 4 Zone Air | Speed D/play | Towbar | 20% Tint | 5mm Castor Bushes | Yellow Calipers, Ceramic Pads |