This pushes the point of boundary-layer separation towards the back on the side of the ball where the spin is moving in the same direction as the surrounding airstream, and towards the front on the side that is moving against the air stream. The result is lower pressure on the side where the boundary layer has become extended, which causes the ball to swing in that direction. That's why a clockwise spin causes the ball to move from left to right. All this assumes that the flow in the boundary layer is laminar, with smooth tiers of air on top of each other. In practice, part of the airflow may be turbulent, with air moving chaotically throughout the layer, and this is where reverse swing can occur.
Experiments show that turbulent layers stick to the surface of the ball longer than laminar layers.
So if the boundary layer is turbulent on one side and laminar on the other, the pressure will be lower on the turbulent side and the ball will swing to that side. Under certain circumstances, turbulence can develop first on the side of the ball which is moving against the airstream, so that the boundary layer here separates later. The result is a reverse swing. Sports such as cricket, which use balls with seams, give bowlers additional opportunities to produce both swing and reverse swing though turbulence. Skilful players can bowl so that the ball spins with its stitched seam always facing at a particular angle to the oncoming air.