umm wiki says.......

Reverse swing is difficult to achieve consistently, as it relies on uneven wear of the ball, tends to occur mostly in hot, dry weather conditions, and requires bowling at high speed. Normal swing can be achieved at relatively moderate bowling speeds, but only the fastest bowlers can regularly produce reverse swing.

Reverse swing tends to be stronger than normal swing, and to occur late in the ball's trajectory. This gives it a very different character to normal swing, and because batsmen experience it less often they generally find it much more difficult to defend against. It is also possible for a ball to swing normally in its early flight, and then to reverse as it approaches the batsman, giving its trajectory an S-shape through the air.

Posted By: DrDublin, Jul 11, 18:02:07