Active D ls used in bright contrasty conditions. It holds more detail in both highlight and shadow areas.
Pictures can appear a little flat but you can pic them up easily in curves.
If you set it to auto when shooting JPGs it can save the day. Its actualy quite efective.
To see it work.. go out in strong sun and take a number of pictures of the same thing, using the different
strenghs. Try 1st without... second on Auto then try the manual fixed settings.
This will give you an idea of how and when it will help.