Well, I was going to say it's a process of lowering the tube down into a volcano where Vulcan and his minions pound on it with giant hammers.... but then I googled it:

"Vulcanization (or vulcanisation) is a chemical process for converting natural rubber or related polymers into more durable materials via the addition of sulfur or other equivalent curatives or accelerators. These additives modify the polymer by forming cross-links (bridges) between individual polymer chains."

But when i say "use a vulcanizing cement" what I really mean is "make sure you use a cement that is specifically formulated for this purpose. Other glues may work... just not as well."