How and Why Ball tampering?

Depending on how the ball is held and released, the new ball swings(i.e. moves in the air) towards the batsman or away from him.When the ball becomes older and gets scuffed up, it moves in the direction of the non-shiny side.The fielders try to keep one side of the ball shiny by applying sweat and saliva.

Fro Example, when bowling an inswinger to a right-handed batsman with an older ball, the bowler holds the ball in such a way that the scuffed-up side faces towards the batsman- then the ball will swing into the batsman. This is conventional swing.

It disappears when the ball gets old.Skilful bowlers then try to reverse-swing the ball : now the ball moves in the direction of the shiny side.

There are two key factors:

• The shiny side of the ball is wetter and heavier due to the constant application of sweat and saliva, while the scuffed up side is dry and lighter.
• The shiny side of the ball, because of its smoothness, offers lesser resistance to the ball than the rough side.

INCIDENTS OF BALL TEMPERING IN HISTORY:-

1. John Lever (1997)
   In January 1997, India captain Bishan singh bedi made a sensational allegation: That England fast bowler John Lever had been applying face cream(Brand:Vaseline) on the ball to shine it. Lever's defence was that he had used coton gauze soaked in vaseline over his eyebrows to stop sweat getting into his eyes, and some traces of the cream had been tranfered to the ball.
   
2. Michael Atherton(1994)
   

   

   
   In July 1994,England captain Michael Atherton was found tampering with the ball by applying dirt he had stored in his pocket.Atherton explained taht he was not changing the condition of the ball,merely maintaining it.When questioned by the referee,he said he was using the dirt to keep his hands dry. Atherton was fined £2000-half for using dirt,half  for lying to the match referee.
3. Rahul Dravid(2004)
   

   

   
   In January 2004 during an India-Zimbabwe ODI, the camera zoomed in at Rahul Dravid and found that he was applying a cough lozenge on the shiny side of the ball. The officials found dravis guilty of ball-tampering and he was fined 40 percent of his match fee.
4. 
   Shahid Afridi
   This is the most bizarre incident on this list- during an ODI against Australia in Perth,Shahis Afridi was caught biting the ball. It's unclear how much damage could he have done to the ball with one sumptuous bite, but he did try.He was banned for two matches

(Source: The Tribune)