Electric Auxiliary Fan Question

[jljljl]

Hey guys,
just a quick question about the aux fan;

I have read many sites online that this fan is supposed to be running at all times as long as the engine is on? Is this true?
I have also read that it only turns on when needed, ie when the engine becomes too hot it would automatically turn on. Or if the a/c was being used?

I tried a couple things to check; I turned the a/c on, put it in N with EBrake on, and checked. The fan was not moving
I also had someone press on the gas (revving engine) while in neutral to see if it would move, still did not.

My question is when is that fan supposed to be moving?
[please bare with me as this is my first time posting a new thread, if there's any tips on making a better thread, let me know!]

2002 bmw x5 e53 3.0i with sports package

the fan i'm speaking of is the one right behind the grilles, looks like this ;


cheers guys