In order to answer this question, it is helpful to look at the title of Chapter 17, which discusses the peopling of the Pacific. That chapter is entitled “Speedboat to Polynesia.” We must ask what the “speedboat” referred to in the title is. The “speedboat” is a sailing canoe with two outriggers. This allowed people to spread out from Southeast Asia across greater expanses of the Pacific. Before the invention of this canoe, people were much more limited because their canoes were not good for crossing great distances of open ocean. This made it more difficult for plants and technology to spread.
Diamond points out that before this invention, people were using single-hulled dugout canoes. He talks about how easy it is for such canoes to tip over. Therefore, they were not well adapted to ocean voyaging. The invention of the double-outrigger sailing canoe was a huge improvement because such canoes are faster and much more seaworthy. On p. 342 in the paperback edition of Guns, Germs, and Steel, Diamond concludes that
The invention of the double-outrigger sailing canoe may have been the technological breakthrough that triggered the Austronesian expansion from the Chinese mainland.