It took the modern science of fluid dynamics to understand exactly what happens in a swerving free kick. When a football moves through the air at low speed the air flow separates from its surface at characteristic points…

When the ball rotates – see graphic 3 – the boundary layer remains tripped but the air flow separation around the ball is distorted. Separation occurs earlier on the side rotating against the flow and later on the side rotating in the same sense as the flow. This causes a pressure differential and a deflecting force which is responsible for moving the ball in the air in a free kick.