How does this work...?

Hi! Okay, this is maybe not the best catagory, but it seems to be the closest I can find. This is something that is baffling me. I've seen people snorkling (on tv) and they have the tube thing that goes in your mouth, and the tube is supposed to stick out of the water so you can breathe. How come sometimes you see people diving right down under the water and they submerge the whole tube? How are they breathing then?? What the heck?