I thought that this second valve was for rapid air deflation but on the wheel that I have it doesn't seem to do anything? I did read somewhere that Porsche made a special pump that could remove the air quickly to enable the collapsible tyre to return to it's un-inflated size correctly but I also read that this second valve was in fact a reverse functioning valve that prevented the spacesaver tyre from being over inflated.
Does anyone have first hand experience with the workings of these or know exactly how it works?