There are two air valves in the nipple that prevent vacuum formation inside the bottle as the milk is being consumed. (babygearhub.net)
The super sensitive air valve is built - in the nipple. (babygearhub.net)
If gunge builds up on the butterfly valve, it can restrict the air required for a smooth idle when the butterfly is closed, so the engine ECU needs to open the Idle Air Valve more that it thinks it should have to and you get into a cycle of surging revs. (mechanics.stackexchange.com)