Use after writing a test, or before trusting one that passed — a test that has never failed is unverified. Do one sabotage run (break the thing under test, confirm the right assertion fails) and confirm each new branch is actually reachable by the mocks/fixtures. Triggers on "test passes", "added a test", "green on first run", "after-the-fact test", "is this test real", "mock", "coverage", and before claiming a behaviour is tested.
