The Quarter seam is the tiny seam which runs around a cricket ball at 90 degrees to the large, raised seam.
While the main seam can be 'picked at' - loosening the threads - in order to create conventional swing when the ball is relatively new, more recently, an understanding has evolved that the quarter seam can be lifted on one side, presumably with the thumb or finger nails, to enable the operation of reverse swing on an already scuffed ball.
Lifting the quarter seam during an over alters the balance of air pressure surrounding the ball as it travels through the air and help it reverse. Unlike picking at the threads of the main seam, the quarter seam, if lifted, can be as easily replaced before the end of the over, when the ball is due to be inspected by the umpires. Umpiring sources in England claim that lifting the quarter seam has been done in the past [1].
Most recently, the fourth Test between England and Pakistan at the Oval in London was forfeited following an accusation of tampering with the ball levelled at the Pakistani team by umpire Darrell Hair, who was observed pointing to the quarter seam of the ball [2] while discussing the issue with his fellow umpire, Billy Doctrove, prior to calling for the ball to be replaced.