Challenge directly means saying the problem plainly and staying with it until the other person has heard it. Scott counts it only when the message gets through.
Bob was the employee she liked too much to correct. He handed in creative work riddled with sloppy mistakes, and she told him it was a great start. That went on for ten months. Looking back, she would point out every problem she saw, and show the whole pattern of mistakes to someone likely to wave it off. Reed Hastings built the keeper test to stop that kind of drift: if you would not fight to keep someone, you tell them early and honestly.
Scott says to start in a neutral place, because going to the outer edge of challenge directly lands you in obnoxious aggression. Go further only if the person brushes you off. Her own boss did this when Scott kept saying um. Scott shrugged off the first mention, then the offer of a speech coach, so her boss kept going until it landed.
Scott calls herself a people pleaser who was raised to be polite. What helped her was seeing that it is unkind not to say the thing.