The venturi effect you're describing is not how spray cans work. It is how (a lot of) spray guns work for airbrishing and stuff. But typical aerosol cans are literally just a substance that's kept liquid by the pressure it's under, with a straw dipped in it leading to a nozzle you can open. That's why once a can gets somewhat empty, you can only get a decent amount of paint/oil/hairspray/etc out if you hold it upright.