I have never quite learned. I've had my car full-on rained in with the thing open (not just tilted). I've opened my door to find the entirety of Autumn in the driver seat. I still forget and leave it open sometimes.I used to tilt it up to keep the car from getting too hot while it was parked. After it rained unexpectedly a few times with it open, I stopped doing that.
Every one of them I’ve had has been too noisy and not enough air circulation to be worth the hassle. Maybe that little fin thing that sticks up wasn’t ever big enough?