The principle
To maintain your independence you must always be needed and wanted. The more relied upon you are, the more freedom you have.
The application
Be the only person who knows the system, holds the relationships, or owns the trust. Power, professionally, is the inverse of how replaceable you are. Document just enough to look generous; never enough to be optional.
The defensive tell — how to spot it being run on you
When a colleague will not document, will not train backup, and routes every related question through themselves, they are building a moat. It is rational; it is also expensive to the team. Decide whether to break the dependency, route around it, or buy it out.