First, the metal goes to the bottom of the mould. At the same time as the metal goes in, the air has to be passed out, either by going out back through the ingate or through the moulds parting line. Most air goes out through the parting line helped by the microscopical channels the talcum powder creates and a slight opening of the two halves allowed by the flexible Prince August clamp. As you are pouring , the metal is increasing the pressure on the air to escape with gravity force. It is therefore imoportant that you fill the mould to the top and also have the metal so hot that it is still liquid in the bottom when it reaches the top of the ingate.
As the moulds do not withstand temperatures over 375°C you can see one of the reasons why the higher melting point alloys do not produce as good a result as lower melting point ones. If you cut airvents, the moulds will fill faster as the air goes out faster, thus making it more suitable for higher melting point alloys. When you cut airvents remember that, once the metal has reached the level of the air vent, it blocks the vent. So you might have to cut several airvents to achieve perfect results. If you cut very fine channels, they will not be seen on your finished casting or destroy any details. An airvent will not reduce the life time of the mould.