Passive research is a mind hack. It’s a way of learning about a specific subject that requires almost no effort. The downside is that it is very long-term.
When you read how to do it, your first instinct will be to scoff. That’s fine. Since it requires so little effort to try it, you can afford to do so no matter how small you expect the payoff to be.
Passively researching something is very similar to simply being interested in it. Here’s the only thing you do differently: make a conscious decision to remember everything you hear relating to the subject. Making that decision makes it easier to remember things; each time the subject comes up, you remember that you’re passively researching it, and automatically start indexing the things you hear with your other knowledge of the subject.
It’s best used to study subjects that for some reaosn aren’t easily researchable by normal means, especially taboo subjects, or for subjects that aren’t important enough to warrent the time investment of actually studying them. It’s also appropriate for gathering information on a subject that has no consensus opinion, which forces you to perform your own, more nuanced opinion survey. Examples include human sexuality, personal information about people you know, or political or religious zeitgeists. Passive research will cause you to learn things that are not easily indexed online or in libraries; expert and less-than-expert opinions and testemonials from people who would not catalog the information in question.