i HAVE IT IN MY X5, iF YOU ARE IN SNOW OR GOING OFF ROADING, yOU PRESS THE BUTTON WHILE GOING DOWN A HILL AND IT WILL TAKE YOU DOWN HILL with a slow speed without you needing to press the gas, its kinda cool but only for snow, ice, mud.
It's on my car... didn't even know I was getting it until the dealer showed me all the features. It's on my dad's X5 too. All it does is help you go down hill at a slow, adjustable speed without pressing the gas or braking.
Most likely you are trying to engage it over 20 mph. The button will not stay on over about 20 mph. A lot of people don't realize how slow that is especially when you are going down a steep hill.
This was a very poor choice for thread revival not only because it was 12 years old but because there is no pertinent information in it. Try this thread which is just from last year and has all the information you need:
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