Explain What Sudoku Difficulty Ratings Actually Mean

Easy, medium, hard and expert sound like a straight line, but two puzzles rated the same can feel completely different depending on which technique they assume you know. This explains what each rating actually measures and shows the one technique that separates an easy grid from a genuinely hard one.

Explain what sudoku difficulty ratings such as easy, medium, hard and expert actually measure: which specific solving techniques each level assumes the solver already knows, and why two puzzles rated the same difficulty can still feel wildly different to solve. Use one plain example of a technique that separates an easy puzzle from a hard one. Assume the reader can solve easy puzzles by scanning but has never learned a named technique.