550 air con/heating system...is it on or off?!?

The air conditioning/heating system in the 550 has a "stop" button on the face of the controls. When this button is depressed, it stays in this position (indented) with the text of "stop" backlighted. When you press it again, the button is now sticking out slightly (un-indented) with the "stop" backlight off.

From my owners manual, I believe the system is OFF when the "stop" button is indented, with the backlight on. I also make sure that the fan is set to zero (not auto) as is the other knob used for airflow direction choice (set to anything but auto...)

Is this a correct assumption?

I had always relied on my interpretation of the manual (practical and understandable owners manuals are a Ferrari weak point to be sure!!) but, on a whim, I tried pressing my stop button when my car was warming up (into the unlit stop button position) and it seemed as if my compressor immediately shut off (based on the sound) This would indicate to me that I had just turned off my system?!?

I know this question may sound strange, but I have never used my system for either heating or cooling, so I havent checked it out by direct usage as of yet....

So, whats the deal on this stop button?