The science of swing is a matter of much debate and, while no one can be exactly certain what is the source on any given day, the end result is sideways movement, often significantly greater than that produced by seam bowlers. It is also generated through the air as opposed to off the pitch like seam bowlers. The key factors in producing swing are the following:
- Uneven wear on the ball - This is achieved by polishing one side of the ball incessantly during the day, while the other side continues to experience wear and tear. The result is a disparity in the aerodynamic qualities of either side.
- Raised seam of the ball - The position of the ball is absolutely pivotal.
- Speed of delivery - Without going into the specific science of turbulent flow as against laminar flow, swing bowling requires reduced pace, with medium fast and fast medium bowlers (and even medium pacers) taking advantage as a result. When combined with a raised seam and differences in the quality of either side of the ball, sideways movement is generated in the direction of the turbulent side.
- Grip on the ball - See below.
- Weather conditions - Particularly significant in England, the weather can improve or damage the chances of swing. Overcast or humid conditions are generally conducive to swing bowling, while beautiful sunshine can render the bowler much less effective.
There are two stock deliveries of the swing bowler which pertain to the grip and the raised seam of the ball:
- Out-swinger - To achieve an out-swinger, the bowler must hold the ball with the rough side placed on the left and grip it with a vertical seam and the first two fingers on either side of it. During the delivery, the bowler then angles the seam slightly left. The result is that the ball swings towards the off-side and away from the right-handed batsman (hence the alternative term ‘away swinger’).
- In-swinger - Again a vertical seam is necessary but the first two fingers must be across the seam angling slightly towards the right (the leg side to a right-handed batsman). The seam must then be angled towards the right before release, giving the ball a chance to move into the right-handed batsman (towards the leg-side).