In order to understand the answer to this question, look at the title of the chapter. It is entitled “Speedboat to Polynesia.” The unique technological advance that allowed the Austronesians to spread out from the Asian mainland was a “speedboat.” Of course, it was not what we now call a speedboat with a motor. Instead, it was 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 “traditional peoples” around the world tend to use dugout canoes for travelling on inland waterways. These canoes are simply made from a tree whose wood has been hollowed out. The resulting canoe is not really very stable at all. There is nothing to stabilize it, so it rolls over very easily. This is not at all good for travelling on the ocean.
Diamond says that Austronesians probably (he does not know for sure) came up with the idea of using outriggers. These were logs that were connected to the hull of the canoe and traveled through the water parallel to, but some way away from, the canoe. If you put one of these on each side of your canoe, you have a much more stable vessel because each outrigger prevents the canoe from tipping in that direction.
Thus, Diamond says that the invention of this kind of canoe “may have been” the advance that allowed Austronesian expansion.