The exact contrary of what is generally believed is often the truth.

Widely held beliefs are not selected for accuracy. They spread because they are easy to say, flattering to the people repeating them, or useful to somebody, and none of those filters favours truth. Sometimes the inversion does turn out to be right, which is startling enough that people build whole personalities on the trick.

The honest use is as a prompt rather than a rule, since automatic contrarianism is only conformity with the sign flipped. Take one thing everybody around you assumes and argue the opposite properly for ten minutes. Most times you will end up back where you started, better informed, and occasionally you will not.