Carl Zeiss Jena Sonnar 180 mm

I've just bought on Ebay a Sonnar 180 mm for my Kiev 60 ,
is the modern version multicoated MC and not the Sonnar zebra .
I can't understand the purpose of a sort of switch that is placed just behind the tripod collar , can be set to position A (in white ) or
M ( in red ).
At first i thought it deserved to detach the basement of the lens and to attach another adapter for different cameras,
but nothing moves if set in position M from A .
If someone knows the lens i'll be grant to know the purpose of that sort of switch button and how to use it .

On most lenses with that it sets the diaphragm to auto or manual mode. In manual mode, it stops down as the f-stop ring is turned. In AUTO mode, it stays open until a pin/tab from the camera presses the same in the lens.

xkaes said:

On most lenses with that it sets the diaphragm to auto or manual mode. In manual mode, it stops down as the f-stop ring is turned. In AUTO mode, it stays open until a pin/tab from the camera presses the same in the lens.

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Yes, it is.
In M works in stop down while in A remains open .
I've never had a lens with this feature before,
many thanks .