The answer to this question can be found in Chapter 17. The chapter is entitled “Speedboat to Polynesia.” Diamond says that the advancement was a “speedboat.” Obviously, Diamond is not referring to a modern motorized speedboat. Instead, he is referring to a double-outrigger sailing canoe. This answer can be found on pages 341 and 342 of the paperback edition of the book.
Diamond says that most people in the world used dugout canoes to travel on inland waterways. These are canoes that are simply made of a single log that is hollowed out. The problem with these canoes is that they are very easily tipped over. There is nothing to prevent them from rolling when they are not in perfect balance. This makes them very bad for ocean voyages.
What the Austronesians apparently came up with was the idea of outriggers. They took two smaller logs and lashed each of them to a framework of poles extending from the main hull of the canoe. Each of the smaller logs was rigged so that it would sit in the water on one side of the main hull, parallel to that hull. This made the new kind of canoe much more stable and much harder to roll over.
Once the Austronesians invented this, it was possible for them to spread out through the Pacific Ocean.