It's not really a secret, it's really just people who not only read the Learning Center but use that information when they create new hubs.
It's really a matter of: research, writing, writing, writing, more research, and a little bit of luck. It's mostly just hard work and not so much luck.
The more hubs you have, the more you can make. Some people make a lot from just one hub and there is even a group of people who make it their goal to make a lot of money from just one hub. Me? I think that is a waste of time and the effort that is put into that can be used to make a lot of hubs which can earn the same or more.
My tip is, do keyword research on things that you're passionate about. Write hubs on the keywords you come up with... mix it up, though... write a hub on one interest, another hub on a completely different interest and see what works. If a particular interest seems to pan out for you, write more hubs on that topic and interlink them. Make the keyword research the extent of your SEO... spend more time writing quality hubs and less time backlinking and you'll find that backlinks come to you naturally. I actually seldom backlink.
I can tell, as a reader, if someone was passionate about something they've written... it really shows.