Underneath the plate is a passage where air goes to the breather hose that plugs into the passenger side valve cover. It is part of the PCV system. There is a gasket under the plate to seal it up. It will also have a film of oil underneath it, unless you put a breather filter at the other end of that hose. Might want to put an airfoil in the front of the TB too if you're taking the cover off. And if your cover didn't come with a new gasket make sure the old one is in good shape, if it doesn't seal tight you will have a vacuum leak.
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