He stepped back and took two more slow swings, feeling the groove he left in the air. Then he stepped up again, set his left leg, fixed his eyes, and wated for the quick swing of the pitcher's arm, the flashing slant of the ball through the blue-and-white air.
Hi! What do these two bolded part mean exactly? Please help!
A groove is a furrow or channel, a long narrow indentation. The batter is feeling the way his bat disturbs the air as he swings. He is imagining or feeling the track of his bat as it moves through the resistance of the air.
From the perspective of a batter, a baseball traveling from the hand of the pitcher (at very high speeds), appears as a white streak. Most pitchers cause the ball to travel with some kind of added movement. In this case the ball is sinking, creating the flashing slant of your quote.