First, let's start by understanding what cross-linking is and the concept of cross-linking. Here, Ouyang Chun lists two forms of cross-linking, progressing from fewer sites involved to more, and so on.
The first type of cross-linking is relatively simple: it's the single triangular cross-linking.
Assume there are three websites: A, B, and C. A links to B - B links to C - C links to A. This forms a triangular cross-linking structure.