The basic principle is that instead of only asking how to achieve a desired outcome, you also ask what would guarantee failure, and then you work to avoid those things. The book traces this approach back to the mathematician Carl Jacobi, who had a habit of solving difficult problems by reversing them, and it also draws heavily on Charlie Munger’s famous line that it is often easier to figure out what you want to avoid than to figure out exactly what you want. The book distinguishes between two related but distinct uses of inversion. The first is a problem-solving technique where you take a forward problem and flip it around to see it from the opposite direction, which can reveal solutions that were not obvious when approached in the usual order. The second is a way of avoiding negative outcomes by focusing on eliminating stupidity and error rather than only chasing brilliance, since removing the sources of failure often does more for your results than adding new sources of success.
Related: Escape the Fourth Quadrant by changing exposure, not prediction