“The angled approach allows for greater use of hip momentum, which creates more rotational momentum—more angular momentum—in the foot,” says William Barfield, a specialist in biomechanics and orthopedic science at the College of Charleston and the Medical University of South Carolina. More angular momentum creates greater foot speed, which delivers more force to the ball.

http://www.scientificamerican.com/article.cfm?id=football-why-do-pro-kickers-use-soccer