Use when the goal is to change code structure without changing behavior — this includes extracting a service object from a fat controller or model, splitting a large class, renaming abstractions, reducing duplication, or reorganizing modules. Covers characterization tests (write tests that document current behavior before touching the code), safe extraction in small steps, and verification after every step. Do NOT use for bug fixes or new features — those follow the TDD gate in rspec-best-practices. Do NOT mix structural changes with behavior changes in the same step.