As I was installing my new hitch, I noticed that the left tailpipe has a 'Butterfly' (valve) that opens and closes. It was closed while idling, I drove it for about an hour and noticed it was still closed? What's it for and when does it open?

I checked my Z4 and it doesn't have that valve.

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Present - 2006 X3
Present - 2003 Z4
Past - 2001 740il
Past - 2001 R1150RT
Past - 2000 Volvo S80