I've always found this help page singularly useless: it explains what the special page is/does, but doesn't talk at all about how to actually use it. Is there anyone with that knowledge who could add documentation here?
Topic on Help talk:Merge history/Flow
Wikipedia has some documentation at wikipedia:Wikipedia:Administrators' guide/Fixing cut-and-paste moves#Using the MergeHistory special page for anyone who wants a non-zero starting point.