or i think you increase the pressure, not sure, havent done it in a while
you can do the same with a flask, water, and a cork with a tube through it at home. water in flask, cork it, reduce pressure with a inverted pump, then quickly pull out the cork, same effect :)
ahhh i get it, cos the higher up you go (lower air pressure) the lower temp that water boils so at -100 pressure it boils lol