Dear Student,
Ballers swing the ball by inclining the seam of the ball at an inclination of 20 - 30o to the direction along which the ball is to be thrown. As a result, major portion of the ball facing tha batsman is smooth. When the ball moves forward, the air flows smoothly around the smooth half but it becomes turbulent on the other side since it has to flow past the seam. The turbulent air is at low pressure than the smooth flowing air. This results in swing the ball sideways.
Regards