Missing info from merging and depupe section of the book

I don't have a very good understanding of dedupe, but I am aware that the section in the book is incomplete. It only seems to talk about using a rule to 'run' a dedupe and merge on existing contacts in the database. I think there is info missing on how dedupe rules are automatically referenced when, for example, a non logged-in user signs up for an event - to check if a similar contact already exists in the database - and updates that contact if there is a match.