That butterfly valve is a secondary intake in case your snow screen clogs with snow, it will draw air from the engine bay which will be warm enough to not have snow/ice clog it up. You'll see its designed to open if the air box gets too much vacuum. It works in conjunction with the snow screen to prevent a build up of snow/ice in the air box. Other things can clog it too, a few good-sized leafs or a bunch of pine needles can also clog up the intake at the snow screen. So, in the event of that happening, there is a secondary intake.
A true CAI (one that draws air from the lowest point possible in the engine bay) will often have a relief mechanism somewhere up near the highest point of the intake system for similar reasons. Those CAI's on a street vehicle also have to deal with the possibility that the intake could become momentarily submerged, though in those situations the relief valve is there to break the vacuum and not suck water directly into the engine.
Your secondary air pump intake should be a hose that was connected to the air box. If you look in the air box as it was installed, on the left-had side there should be a foam looking thingy, which is the filter for the secondary air intake pump hose. The hose would be connected directly beneath it.
Edit: didn't feel my answer was good enough, so added more info about different intake systems.
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