Rumen which houses the microbes, Reticulum also known as many piles, Omasum which helps in absorption of water and nutrients and lastly Abomasum which is the true stomach
It is an involuntary action, as their rumen performs this role to ease movement of food down the digestive tract or for good rumination activities. It's just like we (humans) respire...
But, anything that interferes with that process is life-threatening and that is what we should avoid.
Thanks for such contribution