In the book, what major factor of unification allowed for a smooth flow of technology and crops to all parts of East Asia?
In the book, the major factor of unification that allowed for the smooth flow of technology and crops to all parts of East Asia was geographic in nature.
First, the north-south distances in China (the main exporter of technology, languages, and crops to neighboring Asian regions) were considerably smaller than the ones in the Americas or Africa. China was neither divided by a narrow isthmus (as is Central America) or by desert (as is Africa and Northern Mexico). China's long east-west rivers allowed the diffusion of crops and technology from the coastal harbors to a diverse designation of inland areas.
In addition, China's lack of mountainous terrain allowed the Huang River and the Yangtze River to be joined by canals; this allowed for the efficient diffusion of crops and technology along the northern and southern regions of the country. The south sent iron smelting technology and expertise as well as rice cultivation techniques north. Meanwhile, the north sent knowledge about Sino-Tibetan languages, bronze technology, and state formation to the south. After the reunification of China under the Qin dynasty, its expertise in state formation, food production, technology, and writing was diffused to neighboring Asian countries, notably those in South East Asia.
China's influence on countries in East Asia was tremendous. Even today, the Japanese have opted to preserve the Chinese-derived writing system in its modern literature. So, China's geographic advantage allowed the smooth diffusion of crops and technology to all parts of East Asia, and exchanges between culturally-diverse regions further contributed to advances in Chinese culture and technology.
There are at least two important factors of unification that allowed for this to happen.
First, there were geographic factors. Diamond argues that the topography of East Asia made diffusion relatively easy. He notes that there were long east-west rivers in China along which technology and crops could flow. In addition, there were no major features (like mountain ranges) blocking diffusion along that axis.
Second, there was a political factor. China was so dominant in the area that it created an environment in which technology and crops could spread. Since China created a huge states, there were no political barriers to diffusion of technology and crops.