The mucus migrates towards cold, the lungs being the coolest spot. The pump at the bottom of the lungs starts sending the mud to the sinus and if your lucky and it can drain, down it comes. If its all clogged tight, then your head swells and you actually can throw your skull plates out of place.
Try to not swallow this mucus, it is best to spit it out. All herbs and methods that expells huge amounts of mucus out are good, even if you don't like seeing the yellow mudslide going down the back of your throat.