An inswinger is bowled by holding the cricket ball with the seam vertical and the first two fingers slightly across the seam so that it is angled a little to the leg side. Once the ball has worn and been polished so that one side is rougher than the other, the rough side is placed on the leg side. The ball is placed on the pad of the thumb. This thumb position locks the wrist in a position inclined to the leg side.
Inswing can be bowled from side-on, mid-way or chest on positions. It is the wrist position that is crucial, not the position of hips or shoulders. (See thumb position above).
When the bowler delivers the ball, he angles the seam so that it points slightly to the leg side. To help achieve this position the bowling arm should be near vertical, brushing close to the ear. At release the wrist should remain cocked so as to help impart backspin along the orientation of the seam. The angle of the seam to the direction of motion produces an aerofoil effect as the ball moves through the air, pushing it to the leg side. This is enhanced by differential air pressure caused by movement of air over the rough and smooth surfaces, which also tends to push the ball to the leg side. The result is that the ball curves, or swings in to the batsman.