As we saw in the video, a new cricket ball has two polished sides and both of them are very “smooth” in nature. But, as the ball is being used in the match, it starts to get rough because it is hit by the batsman after every delivery, and also because it hits the ground multiple times during the match. Now, here comes the interesting part. The fielding team continuously applies sweat & saliva on any one particular side of the ball to maintain smoothness on that side. After a particular duration in the match, the ball will look something like this (NOTE: This is only possible if the team chooses to use this particular strategy.)
Now, once the ball has been thrown by the bowler and it is travelling towards the batsman, a thin layer of air (also known as “boundary layer”) is formed around the ball. This layer first wraps around the ball, and then it ultimately tends to escape away from the ball. Carefully observe the picture given below:
When the boundary layer tries to escape away from the ball, it results into an imbalance of forces on the two different sides of the ball i.e. the smooth side and the rough side, which acts as an imbalanced external force for the ball, and it makes the ball swing into a particular direction. When the ball is thrown at a speed 30-70 m/ph, it forms a laminar boundary along the smooth side and a turbulent boundary along the rough side of the ball, and this exact phenomenon is called conventional swing.
Now, you must be wondering that what on Earth laminar and turbulent boundaries are. Well, on the laminar side of the ball, the air particles flow without much disruption. The air particles do not mix with each other and flow in form of parallel lines. On the turbulent side of the ball, the air particles tend to become chaotic and they flow with rapid variation in their speeds. The difference in the flowing style of the air on both the sides causes an imbalance of forces between the two sides, and makes the ball swing.