One way this isn't exactly like Time Machine: since it doesn't hook into the FSEvents journal of file system modifications, rsync has to check the modified date

One way this isn't exactly like Time Machine: since it doesn't hook into the FSEvents journal of file system modifications, rsync has to check the modified dates of every file in the backup set. With FSEvents, Time Machine knows what's changed (or at least which folders contain changed items) and can deal with only those. But, yeah, same basic concept for sure.