When the opening is correctly located facing the base of the windshield, the rain, being somewhat heavier/denser than air, simply doesn't go in-at least when you're moving.
The good ones also have the top edge a bit longer than the bottom-like a roof overhang.
You can also make a 'water trap' in the induction system with a small hole 3mm in it. As (you assume) there will be a higher pressure in the system than out it (the pressure) may help the water out...the theory seems to work for me. I also have a steep rise from the 'trap' area to the induction manifold and also the primary air intake is oblique to the airflow.
