The answer to this can be found in Chapter 1, pages 39 to 41 in the paperback edition. The "Great Leap Forward," to Diamond, is a time during which human beings started to create tools and art that went far beyond anything that had been found up to that point.
Sometime around 50,000 years ago, this "Great Leap" occurred. It can be seen in archaeological sites that have jewelry and standardized stone tools in them. These sites show that humn beings were becoming much more sophisticated in intellectual terms.
It is not known at this point what happened to trigger this great leap, but it is very clear that something happened to allow people to create things that were much more sophisticated than anything that had been created up to that point.