Valve in Exhaust

[9one1]

Can someone please tell me what the valve in the back box of the Exhaust is for

I got a 1997 328i e36

[blackspider]

To make the exhaust quiter during the warm up phase-stops cabin booming

[9one1]

Is that all??

[blackspider]

Yes,nothing else

[9one1]

cool thanks black