In order to achieve this effect, a seam bowler delivers the ball with the seam held upright, holding the ball with his thumb and middle & index finger.
As he releases the ball, the two fingers roll the ball backwards. This keeps the seam aligned vertically as it travels towards the batsman, making it possible for the ball to bounce on with the seam upright on the pitch.
The direction and degree of deviation from a straight path are dependent on the small-scale alignment of the seam and any irregularities in the pitch surface.
This means that deviation caused by seam is chaotic and unpredictable.
Often the deviation caused by seam is not large enough to cause a batsman significant problems with playing the ball.