With the cricket season in full flight, you may find yourself asking an increasingly common question: just what is reverse swing?
Most cricket fans are broadly familiar with the dark arts of swing bowling. But the practice is replete with myriad complexities, and the reverse-swing discipline in particular can seem perplexing to the uninitiated.
According to Nasa scientist Rabindra Metha, the explanation for why cricket balls swing is to be found in flow physics. As the ball moves through the air, it is effectively cushioned by a "boundary layer", which separates from one part of the ball but not the other. The separation point creates a pressure system. If there are different levels of pressure on either side of a ball, it will swing while in flight.
While hard and relatively new, the ball will swing in a predictable fashion. Cricketers, and fast bowlers in particular, are adept at keeping one side of the ball shiny. A bowler bowling to a right-handed batsman can get the ball to swing towards him if he points the seam of the ball towards fine leg with the shiny side positioned to the left.
The ball swings towards the batsman in the direction of the non-shiny side because the air pressure is different on either side of the ball. In aerodynamic terms, the shiny side has laminar air flowing past it, while the non-shiny side is affected by turbulent airflow.
Reverse swing comes into play as the ball gets older. The reason it can be so devilish for batsmen to play is that the ball swings the other way (towards the shiny side rather than away from it) even though the bowler does nothing differently in his action.
Metha explains that the drier and rougher the non-shiny side becomes, the pressure systems affecting the ball change. The laminar layer around the shiny side separates earlier, turning the airflow turbulent. And the turbulent layer around the rough side also separates earlier than in conventional swing and, importantly, earlier than the new turbulence around the shiny side.
Cricket historian Peter Oborne traces reverse swing's ancestry to Pakistan, and in particular to Sarfraz Nawar, who played for Pakistan during the 1970s and '80s.
According to Oborne, Sarfraz developed the technique at Lahore's Mozang Link Cricket Club, where, like all great innovations, it came to him suddenly.
"One day, I shone one side of a very old ball," said Sarfraz. "It was rough on both sides but I shone one side and it swung towards the shine - it should not have done this."
From there, the cunning behind reverse took on the quality of a state secret for passing generations of Pakistani bowlers, from Sarfraz to Imran Khan to Wasim Akram and Waqar Younis.