ChokeOnSmoke is 100% correct!

The nasal passage and throat is blocked when engulfing a prey item, and since the trachea is constructed of a tube with hard rings of cartilage around it, it is always open, so they usually breathe via their glottis while eating/swallowing prey.
Heather is also correct when she says that they push it out to breathe when eating, at most times it is collapsed, you could say it's somewhat retractable, and yes they also use it to hiss.