What does this button do?
 

http://img.tapatalk.com/425779ab-aebc-cf2e.jpg with the down hill and the speedo on it?


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I am here: http://maps.google.com/maps?ll=30.221074,-95.564268

That would be the button which activates the hill descent feature on your X5.

Hill Descent allows a smooth and controlled hill descent in rough terrain without the driver needing to touch the brake pedal. Push the button and the vehicle will descend using the ABS brake system to control each wheel's speed. If the vehicle accelerates without driver input, the system will automatically apply the brakes to slow down to the desired vehicle speed.

ECS Tuning

And you can control the descent speed with cruise +/- buttons.