Originally Posted by FormerRotor
I'm obviously not a BMW design engineer, but it seems to me that the spring was likely selected to be compressed at a certain negative pressure to allow air into the box from the engine compartment. This would likely be useful if the intake were clogged so that the engine is not choked out.
Correct. If the intake gets clogged with snow, the other warm air intake opens and lets air in. I believe VW does this too.