Contrast swing is achieved when the seam of the ball is released straight up instead of being angled towards slip or fine leg. Here, the ball will swing when there is a contrast in surface conditions of two sides of the ball. The greater the difference between the rough side and the smooth side, the more the tendency of the ball to alter its direction.

In contrast swing, the direction of the swing of the cricket ball is determined by the speed at which the ball is being delivered and the condition of the boundary layer of the ball instead of its seam and surfaces. The critical bowling speed which determines the direction of the swing of the ball is a factor of the condition of the ball.

The scientific principle behind contrast swing.

When the ball is delivered at a relatively low speed, the boundary layer over the upper surface separates relatively earlier in a laminar while that on the bottom rough side becomes turbulent and separates later. This asymmetry of high- and low-pressure regions results in a side force which makes the ball swing towards the rough side. In competitive cricket, this is often spotted when part time bowlers come on to chance their arms. If the ball is delivered at a comparatively higher speed, the transition occurs on both sides, but there is less turbulence and thus higher pressure on the rough side. Consequently, the boundary layer on the rough side separates relatively early and the ball swings towards the smooth side.

Contrast swing is often observed when part time medium pacers come on to chance their arm.

The principles followed by contrast swing are same as reverse swing and therefore, it is often misinterpreted as reverse swing. Contrast swing relies on the contrast in the roughness between the two sides that results in the asymmetry in airflow between the two sides of the ball, and hence the swing. On the other hand, reverse swing happens when the ball swings against the direction of the seam, with the non-seam side playing a huge factor in its critical speed. One big and unique advantage of contrast swing is that unlike reverse swing and conventional swing, it doesn’t depends on the condition of the seam of the ball and therefore it can be achieved even when the seam of the ball is damaged or flattened provided that there is a contrast between the two surfaces of the ball.

Shami bowling with the old ball is as good an exhibition of contrast swing as one can get.