When a soccer player kicks a ball off-center it causes the ball to spin. The direction and speed of the spin will determine how much the ball curves during flight. It's the same principle as a curve ball in baseball. When throwing the ball, the pitcher makes a fast spin which causes the ball to curve during flight. As the ball spins, friction between the ball and air causes the air to react to the direction of spin of the ball.
As the ball undergoes top-spin it causes the velocity of the air around the top half of the ball to become less than the air velocity around the bottom half of the ball. This is because the tangential velocity of the ball in the top half acts in the opposite direction to the airflow, and the tangential velocity of the ball in the bottom half acts in the same direction as the airflow. The airflow is in the leftward direction, relative to the ball.
Since the (resultant) air speed around the top half of the ball is less than the air speed around the bottom half of the ball, the pressure is greater on the top of the ball. This causes a net downward force (F) to act on the ball. This is due to Bernoulli's principle which states that when air velocity decreases, air pressure increases (and vice-versa).
In 1997, Brazilian player Roberto Carlos rendered a French keeper stunned and speechless after curving a 30 meter strike off the post and into the goal. The Magnus effect explains how projectiles can curve when moving through a fluid (like air). Whenever a ball is spinning through the air, the Magnus "force" will push it in a direction perpendicular to the direction of movement. After Carlos sent the ball flying, the airflow started pushing against the ball in the direction of Carlos. This means that the side of the ball spinning toward Carlos would move with the airflow while the opposite side would move against it. This imbalance is the key to the Magnus effect. The airflow moving with the