Answer: The high ones
Prior to meeting with the baseball scout, Kit is shown striking out, after missing three high fastballs, despite being warned by Dottie to "Lay off those high ones". Much later on in the film, when Kit and Dottie are no longer speaking, and find themselves on opposing teams, and in the World Series, Dottie tries to use this weakness against her. For when Kit gets up to hit, Dottie informs the pitcher, Ellen Sue, of Kit's weakness for highballs. Expecting her to strike out, Dottie is flabbergasted when, on the last pitch, Kit hits the ball, and, as a result, goes on to win the game for her team, the Racine Belles.