The term 'Progressive Pitcher' refers to a pitcher who uses a variety of pitches to keep the batter guessing. Flo, the three letter answer to this crossword, is a slang term for a fastball, which is one of the most common pitches used by progressive pitchers. Progressive pitchers use a combination of fastballs, curveballs, sliders, and changeups to keep batters off balance and increase their chances of success.