Reverse Swing

Wikipedia:

Origin

Former Pakistan international Sarfraz Nawaz was the founder of reverse swing during the late 1970s, and he passed his knowledge on to former team-mate and captain Imran Khan. It was Imran who schooled bowlers Wasim Akram and Waqar Younis, who brought the art to the cricket world's attention during the late 1980s and 1990s. The dynamic duo managed to make the old ball swing a considerable distance at pace in both directions, a skill few bowlers can master.
Khan Mohammad is also known as a possible initiator of this art who then passed it on to Sarfraz Nawaz and others.
In a display of reverse swing in a Test match against Australia in 1979 in Melbourne, Sarfraz Nawaz took nine wickets in an innings. This included a remarkable spell of 33 deliveries in which he captured 7 wickets for 1 run. This is when the cricket world noticed this new form of fast bowling.
Wasim Akram then brought reverse swing to the public limelight but the man who really put the reverse into swing was Waqar Younis. He bucked the 1980s trend of pitching fast and short by pitching fast and full. Not an obvious recipe for success until the prodigious late inswing is factored in, which was designed to smash into the base of leg stump or the batsman's toes.
[edit]How does it work?

There have been plenty of theories about why, but here's an explanation from former England bowling coach Troy Cooley:
"Reverse swing is all to do with the deterioration of the ball and the seam position in flight. As the ball becomes rougher, it will take on a different characteristic as it deteriorates. So if you present the ball as an outswinger, the ball has deteriorated so much on the rough side that it takes on the characteristics of the shiny side. Which means a natural outswinger will become an inswinger and conversely, an inswinger into an outswinger."
The defining point of swing is the separation point of the initial layer of air with the ball, whichever side has greater air coverage will have more lift and lower pressure, lifting and sucking the ball towards it. For greater detail on what causes separation-point differences see swing bowling.